Foreword
Most research projects never leave the academic environment. Occasionally, exceptional ones
survive the transition from the university to the real world and go on to become a phenomenon.
POSTGRESQL is one of those projects. Its popularity and success are a testament to the dedication
and hard work of the POSTGRESQL global development team. Although developing an advanced
database system is no small feat, maintaining and enhancing an inherited code base are even more
challenging. The POSTGRESQL team has managed to not only improve the quality and usability of
the system, but also expand its use among the Internet user community. This book marks a major
milestone in the history of the project.
Postgres95, later renamed POSTGRESQL, started as a small project to overhaul Postgres.
Postgres was a novel and feature-rich database system created by the students and staff at the
University of California at Berkeley. Our goal with Postgres95 was to keep the powerful and
useful features of this system while trimming down the bloat caused by much experimentation
and research. We had a lot of fun reworking the internals. At the time, we had no idea where
we were going with the project. The Postgres95 exercise was not research, but simply a bit of
engineering housecleaning. By the spring of 1995 however, it had occurred to us that the Internet
user community really needed an open source, SQL-based multiuser database. Happily, our first
release was met with great enthusiasm, and we are very pleased to see the project continuing.
Obtaining information about a complex system like POSTGRESQL is a great barrier to its
adoption. This book fills a critical gap in the documentation of the project and provides an excellent
overview of the system. It covers a wide range of topics, from the basics to the more advanced
and unique features of POSTGRESQL.
In writing this book, Bruce Momjian has drawn on his experience in helping beginners with
POSTGRESQL. The text is easy to understand and full of practical tips. Momjian captures database
concepts using simple and easy-to-understand language. He also presents numerous real-life
examples throughout the book. In addition, he does an outstanding job of covering many advanced
POSTGRESQL topics. Enjoy reading the book and have fun exploring POSTGRESQL! It is our hope
this book will not only teach you about using POSTGRESQL, but also inspire you to delve into its
innards and contribute to the ongoing POSTGRESQL development effort.
Jolly Chen and Andrew Yu, co-authors of Postgres95
xxiii

Preface
This book is about POSTGRESQL, the most advanced open source database. From its origins in
academia, POSTGRESQL has moved to the Internet with explosive growth. It is hard to believe the
advances during the past four years under the guidance of a team of worldwide Internet developers.
This book is a testament to their vision, and to the success that POSTGRESQL has become.
The book is designed to lead the reader from their first database query through the complex
queries needed to solve real-world problems. No knowledge of database theory or practice is
required. However, basic knowledge of operating system capabilities is expected, such as the
ability to type at an operating system prompt.
Beginning with a short history of POSTGRESQL, the book moves from simple queries to the
most important database commands. Common problems are covered early, which should prevent
users from getting stuck with queries that fail. The author has seen many bug reports in the past
few years and consequently has attempted to warn readers about the common pitfalls.
With a firm foundation established, additional commands are introduced. The later chapters
outline complex topics like transactions and performance.
At each step, the purpose of each command is clearly illustrated. The goal is to have readers
understand more than query syntax. They should know why each command is valuable, so they
can use the proper commands in their real-world database applications.
A database novice should read the entire book, while skimming over the later chapters. The
complex nature of database systems should not prevent readers from getting started. Test
databases offer a safe way to try queries. As readers gain experience, later chapters will begin to make more sense. Experienced database users can skip the early chapters on basic SQL
functionality. The cross-referencing of sections allows you to quickly move from general to more
specific information.
Much information has been moved out of the main body of the book into appendices. Appendix A
lists sources of additional information about POSTGRESQL. Appendix B provides information about
installing POSTGRESQL. Appendix C lists the features of POSTGRESQL not found in other database
systems. Appendix D contains a copy of the POSTGRESQL manual pages which should be consulted
anytime you have trouble with query syntax. Also, do not overlook the excellent documentation
that is part of POSTGRESQL. This documentation covers many complex topics, including much
POSTGRESQL-specific functionality that cannot be covered in a book of this length. Sections of the
xxv

xxvi

PREFACE

documentation are referenced in this book where appropriate.
This book uses italics for identifiers, SMALLCAPS for SQL keywords, and a monospaced font for
SQL queries. The Web site for this book is located at http://www.postgresql.org/docs/awbook.html .

Acknowledgments
POSTGRESQL and this book would not be possible without the talented and hard-working members
of the POSTGRESQL Global Development Team. They took source code that could have become
just another abandoned project and transformed it into the open source alternative to commercial
database systems. POSTGRESQL is a shining example of Internet software development.

Steering
• Fournier, Marc G., in Wolfville, Nova Scotia, Canada, coordinates the entire effort, provides
the server, and administers the primary Web site, mailing lists, ftp site, and source code
repository.
• Lane, Tom, in Pittsburgh, Pennsylvania, United States, is often seen working on the planner/optimizer, but has left his fingerprints in many places. He specializes in bug fixes and
performance improvements.
• Lockhart, Thomas G., in Pasadena, California, United States, works on documentation, data
types (particularly date/time and geometric objects), and SQL standards compatibility.
• Mikheev, Vadim B., in San Francisco, California, United States, does large projects, like
vacuum, subselects, triggers, and multi-version concurrency control (MVCC).
• Momjian, Bruce, in Philadelphia, Pennsylvania, United States, maintains FAQ and TODO lists,
code cleanup, patch application, training materials, and some coding.
• Wieck, Jan, near Hamburg, Germany, overhauled the query rewrite rule system, wrote our
procedural languages PL/PGSQL and PL/TCL, and added the NUMERIC type.

Major Developers
• Cain, D’Arcy, J. M., in Toronto, Ontario, Canada, worked on the
and the INET type.

• Eisentraut, Peter, in Uppsala, Sweden, has added many features, including an overhaul of
psql.
• Elphick, Oliver, in Newport, Isle of Wight, United Kingdom, maintains the POSTGRESQL
package for Debian Linux.
• Horak, Daniel, near Pilzen, Czech Republic, did the WinNT port of POSTGRESQL (using the
Cygwin environment).
• Inoue, Hiroshi, in Fukui, Japan, improved btree index access.
• Ishii, Tatsuo, in Zushi, Kanagawa, Japan, handles multibyte foreign language support and
porting issues.
• Martin, Dr. Andrew C. R., in London, United Kingdom, created the ECPG interface and helped
in the Linux and Irix FAQs including some patches to the POSTGRESQL code.
• Mergl, Edmund, in Stuttgart, Germany, created and maintains pgsql_perl5. He also created
DBD-Pg, which is available via CPAN.
• Meskes, Michael, in Dusseldorf, Germany, handles multibyte foreign language support and
maintains ECPG.
• Mount, Peter, in Maidstone, Kent, United Kingdom, created the Java JDBC interface.
• Nikolaidis, Byron, in Baltimore, Maryland, United States, rewrote and maintains the ODBC
interface for Windows.
• Owen, Lamar, in Pisgah Forest, North Carolina, United States, maintains the RPM package.
• Teodorescu, Constantin, in Braila, Romania, created the PGACCESS interface.
• Thyni, Göran, in Kiruna, Sweden, has worked on the Unix socket code.

POSTGRESQL is the most advanced open source database server. In this chapter, you will learn
about databases, open source software, and the history of POSTGRESQL.
Three basic office productivity applications exist: word processors, spreadsheets, and databases.
Word processors produce text documents critical to any business. Spreadsheets are used for financial
calculations and analysis. Databases are used primarily for data storage and retrieval. You can use a
word processor or spreadsheet to store small amounts of data. However, with large volumes of data
or data that must be retrieved and updated frequently, databases are the best choice. Databases
allow orderly data storage, rapid data retrieval, and complex data analysis.

1.2

University of California at Berkeley

POSTGRESQL’S ancestor was Ingres, developed at the University of California at Berkeley (1977–
1985). The Ingres code was later enhanced by Relational Technologies/Ingres Corporation, 1 which
produced one of the first commercially successful relational database servers. Also at Berkeley,
Michael Stonebraker led a team to develop an object-relational database server called Postgres
(1986–1994). Illustra2 took the Postgres code and developed it into a commercial product. Two
Berkeley graduate students, Jolly Chen and Andrew Yu, subsequently added SQL capabilities to
Postgres. The resulting project was called Postgres95 (1994–1995). The two later left Berkeley,
but Chen continued maintaining Postgres95, which had an active mailing list.
1 Ingres

Corporation was later purchased by Computer Associates.
was later purchased by Informix and integrated into Informix’s Universal Server.

2 Illustra

1

CHAPTER 1. HISTORY OF POSTGRESQL

2

1.3

Development Leaves Berkeley

In the summer of 1996, it became clear there was great demand for an open source SQL database
server, and a team formed to continue development. Marc G. Fournier of Toronto, Canada, offered
to host the mailing list and provide a server to host the source tree. One thousand mailing list
subscribers were moved to the new list. A server was configured, giving a few people login
accounts to apply patches to the source code using cvs.3
Jolly Chen has stated, "This project needs a few people with lots of time, not many people with
a little time." Given the 250,000 lines of C4 code, we understood what he meant. In the early
days, four people were heavily involved: Marc Fournier in Canada; Thomas Lockhart in Pasadena,
California; Vadim Mikheev in Krasnoyarsk, Russia; and me in Philadelphia, Pennsylvania. We all
had full-time jobs, so we participated in the effort in our spare time. It certainly was a challenge.
Our first goal was to scour the old mailing list, evaluating patches that had been posted to fix
various problems. The system was quite fragile then, and not easily understood. During the first
six months of development, we feared that a single patch might break the system and we would
be unable to correct the problem. Many bug reports left us scratching our heads, trying to figure
out not only what was wrong, but how the system even performed many functions.
We had inherited a huge installed base. A typical bug report came in the following form: "When
I do this, it crashes the database." We had a long list of such reports. It soon became clear that
some organization was needed. Most bug reports required significant research to fix, and many
reports were duplicates, so our TODO list included every buggy SQL query. This approach helped
us identify our bugs, and made users aware of them as well, thereby cutting down on duplicate bug
reports.
Although we had many eager developers, the learning curve in understanding how the database
worked was significant. Many developers became involved in the edges of the source code, like
language interfaces or database tools, where things were easier to understand. Other developers
focused on specific problem queries, trying to locate the source of the bug. It was amazing to
see that many bugs were fixed with just one line of C code. Because Postgres had evolved in an
academic environment, it had not been exposed to the full spectrum of real-world queries. During
that period, there was talk of adding features, but the instability of the system made bug fixing our
major focus.

1.4

POSTGRESQL Global Development Team

In late 1996, we changed the name of the database server from Postgres95 to POSTGRESQL. It is a
mouthful, but honors both the Berkeley name and its SQL capabilities. We started distributing the
source code using remote cvs, which allowed people to keep up-to-date copies of the development
tree without downloading an entire set of files every day.
3 cvs

4C

sychronizes access by developers to shared program files.
is a popular computer language first developed in the 1970s.

1.4. POSTGRESQL GLOBAL DEVELOPMENT TEAM

3

Releases occurred every three to five months. Each period consisted of two to three months
of development, one month of beta testing, a major release, and a few weeks to issue sub-releases
to correct serious bugs. We were never tempted to follow a more aggressive schedule with more
releases. A database server is not like a word processor or game, where you can easily restart it if
a problem arises. Instead databases are multiuser, and lock user data inside the database, so they
must be as reliable as possible.
Development of source code of this scale and complexity is not for the novice. We initially had
trouble interesting developers in a project with such a steep learning curve. However, over time,
our civilized atmosphere and improved reliability and performance helped attract the experienced
talent we needed.
Getting our developers the knowledge they needed to assist with POSTGRESQL was clearly a
priority. We had a TODO list that outlined what needed to be done, but with 250,000 lines of code,
taking on any item was a major project. We realized developer education would pay major benefits
in helping people get started. We wrote a detailed flowchart of the database modules. 5 We also
wrote a developers’ FAQ,6 answering the most common questions of POSTGRESQL developers.
With this information, developers became more productive at fixing bugs and adding features.
Although the source code we inherited from Berkeley was very modular, most Berkeley coders
used POSTGRESQL as a test bed for research projects. As a result, improving existing code was
not a priority. Their coding styles were also quite varied.
We wrote a tool to reformat the entire source tree in a consistent manner. We wrote a script to
find functions that could be marked as static7 or unused functions that could be removed completely.
These scripts are run just before each release. A release checklist reminds us of the items to be
changed for each release.
As we gained knowledge of the code, we were able to perform more complicated fixes and
feature additions. We redesigned poorly structured code. We moved into a mode where each
release had major new features, instead of just bug fixes. We improved SQL conformance, added
sub-selects, improved locking, and added missing SQL functionality. A company was formed to
offer telephone support.
The Usenet discussion group archives started touting us. At one time, we had searched for
POSTGRESQL and found that many people were recommending other databases, even though
we were addressing user concerns as rapidly as possible. One year later, many people were
recommending us to users who needed transaction support, complex queries, commercial-grade
SQL support, complex data types, and reliability—clearly our strengths. Other databases were
recommended when speed was the overriding concern. Red Hat’s shipment of POSTGRESQL as
part of its Linux8 distribution quickly expanded our user base.
Today, every release of POSTGRESQL is a major improvement over the last. Our global
5 All

the files mentioned in this chapter are available as part of the POSTGRESQL distribution,

http://www.postgresql.org/docs.
6 Frequently

Asked Questions
static function is used by only one program file.
8 Linux is a popular UNIX-like, open source operating system.
7A

or at

CHAPTER 1. HISTORY OF POSTGRESQL

4

development team has mastery of the source code we inherited from Berkeley. In addition, every
module is understood by at least one development team member. We are now easily adding major
features, thanks to the increasing size and experience of our worldwide development team.

1.5

Open Source Software

POSTGRESQL is open source software. The term “open source software” often confuses people.
With commercial software, a company hires programmers, develops a product, and sells it to
users. With Internet communication, however, new possibilities exist. Open source software has
no company. Instead, capable programmers with interest and some free time get together via
the Internet and exchange ideas. Someone writes a program and puts it in a place everyone can
access. Other programmers join and make changes. When the program is sufficiently functional,
the developers advertise the program’s availability to other Internet users. Users find bugs and
missing features and report them back to the developers, who, in turn, enhance the program.
It sounds like an unworkable cycle, but in fact it has several advantages:
• A company structure is not required, so there is no overhead and no economic restrictions.
• Program development is not limited to a hired programming staff, but taps the capabilities
and experience of a large pool of Internet programmers.
• User feedback is facilitated, allowing program testing by a large number of users in a short
period of time.
• Program enhancements can be rapidly distributed to users.

1.6

Summary

This chapter has explored the long history of POSTGRESQL, starting with its roots in university
research. POSTGRESQL would not have achieved its success without the Internet. The ability
to communicate with people around the world has allowed a community of unpaid developers to
enhance and support software that rivals commercial database offerings. By allowing everyone
to see the source code and contribute to its ongoing development, POSTGRESQL continues to
improve every day. The remainder of this book shows how to use this amazing piece of software.

Chapter 2

Issuing Database Commands
In this chapter, you will learn how to connect to the database server and issue simple commands
to the POSTGRESQL server.
At this point, the book makes the following assumptions:
• You have installed POSTGRESQL.
• You have a running POSTGRESQL server.
• You are configured as a POSTGRESQL user.
• You have a database called test.
If not, see Appendix B.

2.1

Starting a Database Session

POSTGRESQL uses a client/server model of communication. A POSTGRESQL server is continually
running, waiting for client requests. The server processes the request and returns the result to
the client.

Choosing an Interface
Because the POSTGRESQL server runs as an independent process on the computer, a user cannot
interact with it directly. Instead, client applications have been designed specifically for user
interaction. This chapter describes how to interact with POSTGRESQL using the psql client
application. Additional interfaces are covered in Chapters 16 and 17.

\copyright for distribution terms
\h for help with SQL commands
\? for help on internal slash commands
\g or terminate with semicolon to execute query
\q to quit

test=>

Figure 2.1: psql session start-up

Choosing a Database
Each POSTGRESQL server controls access to a number of databases. Databases are storage areas
used by the server to partition information. For example, a typical installation may have a production
database, used to keep all information about a company. It may also have a training database, used
for training and testing purposes. They may have private databases, used by individuals to store
personal information. For this exercise, we will assume that you have created an empty database
called test. If not, see Appendix B.

Starting a Session
To start a psql session and connect to the test database, type psql test at the command prompt.
Your output should look similar to Figure 2.1. Remember, the operating system command prompt
is case-sensitive, so you must type this in all lowercase. 1

2.2

Controlling a Session

Congratulations. You have successfully connected to the POSTGRESQL server. You can now issue
commands and receive replies from the server. Letâ&#x20AC;&#x2122;s try one. Type SELECT CURRENT_USER; and
press Enter (see Figure 2.2). If you make a mistake, just press Backspace and retype the command.
It should show your login name underneath the dashed line. This example shows the login name of
postgres. The word getpgusername is a column label. The server also reports that it has returned
one row of data. The line test=> tells you that the server has finished its current task and is waiting
for the next database query.
1A

Let’s try another one. At the test=> prompt, type SELECT CURRENT_TIMESTAMP; and press Enter.
You should see the current date and time. Each time you execute the query, the server will report
the current time to you.

Typing in the Query Buffer
Typing in the query buffer is similar to typing at an operating system command prompt. However,
at an operating system command prompt, Enter completes each command. In psql, commands are
completed only when you enter a semicolon (;) or backslash-g (\g).
As an example, let’s do SELECT 1 + 3; but in a different way. See Figure 2.3.2 Notice that the
query is spread over three lines. The prompt changed from => on the first line to -> on the second
line to indicate that the query was continued. The semicolon told psql to send the query to the
server. We could have easily replaced the semicolon with backslash-g. I do not recommend that
you type queries as ugly as this one, but longer queries will benefit by being spread over multiple
2 Don’t

lines. You might notice that the query is in uppercase. Unless you are typing a string in quotes,
the POSTGRESQL server does not care whether words are uppercase or lowercase. For clarity, I
recommend you enter words special to POSTGRESQL in uppercase.
Try some queries on your own involving arithmetic. Each computation must start with the
word SELECT, then your computation, and finally a semicolon or backslash-g. For example, SELECT
4 * 10; would return 40. Addition is performed using a plus symbol (+), subtraction using a minus
symbol (-), multiplication using an asterisk (*), and division using a forward slash (/).
If you have readline3 installed, psql will even allow you to use your arrow keys. Your left and
right arrow keys allow you to move around, and the up and down arrows retrieve previously typed
queries.

Displaying the Query Buffer
You can continue typing indefinitely, until you use a semicolon or backslash-g. Everything you type
will be buffered by psql until you are ready to send the query. If you use backslash-p (\p), you will
see everything accumulated in the query buffer. In Figure 2.4, three lines of text are accumulated
and displayed by the user using backslash-p. After display, we use backslash-g to execute the query,
which returns the value 21. This ability comes in handy with long queries.

Erasing the Query Buffer
If you do not like what you have typed, use backslash-r (\r) to reset or erase the buffer.
3 Readline

is an open source library that allows powerful command-line editing.

2.3. GETTING HELP

2.3

9

Getting Help

You might ask, â&#x20AC;&#x153;Are these backslash commands documented anywhere?â&#x20AC;? If you look at Figure 2.1,
you will see that the answer is printed every time psql starts. Backslash-? (\?) prints all valid
backslash commands. Backslash-h displays help for SQL commands. SQL commands are covered
in the next chapter.

2.4 Exiting a Session
This chapter would not be complete without showing you how to exit psql. Use backslash-q (\q)
to quit the session and exit psql. Backslash g (go), p (print), r (reset), and q (quit) should be all you
need for now.

2.5 Summary
This chapter has introduced the most important features of psql. This knowledge will allow you
to try all the examples in this book. In addition, psql has many other features to assist you.
Section 16.1 covers psql in detail. You may want to consult that chapter while reading through the
book.

Chapter 3

Basic SQL Commands
SQL stands for Structured Query Language. It is the most common way to communicate with
database servers, and is supported by almost all database systems. In this chapter, you will learn
about relational database systems and how to issue the most important SQL commands.

3.1

Relational Databases

As mentioned in Section 1.1, the purpose of a database is rapid data storage and retrieval. Today,
most database systems are relational databases. While the term “relational database” has a mathematical foundation, in practice it means that all data stored in the database is arranged in a uniform
structure.
Figure 3.1 shows a database server with access to three databases: demo, finance, and test.
You could issue the command psql finance and be connected to the finance database. You have
already dealt with this issue in Chapter 2. Using psql, you chose to connect to database test with
the command psql test. To see a list of databases available at your site, type psql -l. The first
column lists the database names. However, you may not have permission to connect to all of them.
You might ask, “What are those black rectangles in the databases?” They are tables. Tables are
the foundation of a relational database management system ( RDBMS). They hold the data stored in a
database. Each table has a name defined by the person who created it.
Let’s look at a single table called friend shown in Table 3.1. You can readily see how tables
are used to store data. Each friend is listed as a separate row in the table. The table records five
pieces of information about each friend: firstname, lastname, city, state, and age. 1
Each friend appears on a separate row; each column contains the same type of information.
This is the type of structure that makes relational databases successful. It allows you to select
certain rows of data, certain columns of data, or certain cells. You could select the entire row for
Mike, the entire column for City, or a specific cell like Denver.
1 In a real-world database, the person’s birth date would be stored and not the person’s age. The age must be updated each
time the person has a birthday. A person’s age can be computed when needed from a birth date field.

Some synonyms exist for the terms “table,” “row,” and “column.” “Table” is more formally
referred to as a relation or class, “row” as record or tuple, and “column” as field or attribute.

3.2

Creating Tables

Let’s create our own table and call it friend. Figure 3.2 shows the psql statement to create this
table. You do not have to type the command exactly this way. You can use all lowercase, or you
can write it in one long line, and it would work just the same.
Let’s look at the statement from the top down. The words CREATE TABLE have special meaning
to the database server. They indicate that the next request from the user is to create a table. You
will find most SQL requests can be quickly identified by the first few words. The rest of the request
has a specific format that is understood by the database server. While capitalization and spacing
are optional, the format for a query must be followed exactly. Otherwise, the database server
will issue an error such as parser: parse error at or near "pencil", meaning that the database
server became confused near the word pencil. In such a case, the manual page for the command
should be consulted and the query reissued in the proper format. A copy of the POSTGRESQL
manual pages appears in Appendix D.
The CREATE TABLE command follows a specific format: first, the two words CREATE TABLE; then
the table name; then an opening parenthesis; then a list of column names and their types; followed
by a closing parenthesis. The important part of this query appears between the parentheses. You
will notice five lines there in Figure 3.2. The first line, firstname CHAR(15), represents the first
column of the table to create. This column is named firstname, and the text CHAR(15) indicates
the column type and length. The CHAR(15) means the column holds a maximum of 15 characters.
The second column is called lastname and holds a maximum of 20 characters. Columns of type
CHAR() hold characters of a specified length. User-supplied character strings 2 that do not fill the
2A

entire length of the field are right-padded with blanks. The columns city and state are similar. The
final column, age, is different, however. It is not a CHAR() column, but rather an INTEGER column.
It holds whole numbers, not characters. Even if the table contained 5,000 friends, you could be
certain that no names appeared in the age column, only whole numbers. This consistent structure
helps databases to be fast and reliable.
POSTGRESQL supports more column types than just CHAR() and INTEGER. However, in this
chapter we will use only these two. Sections 4.1 and 9.2 cover column types in more detail.
Create some tables yourself now. Use only letters for your table and column names. Do not
use any numbers, punctuation, or spaces at this time.
The \d command allows you to see information about a specific table or to list all table names
in the current database. To see information about a specific table, type \d followed by the name
of the table. For example, to see the column names and types of your new friend table in psql,
type \d friend (Figure 3.3). If you use \d with no table name after it, you will see a list of all table
names in the database.

3.3

Adding Data with INSERT

Letâ&#x20AC;&#x2122;s continue toward the goal of making a table exactly like the friend table shown in Table 3.1.
So far, we have created the table, but it does not contain any friends. You add rows into a table with
the INSERT statement. Just as CREATE TABLE has a specific format that must be followed, INSERT
also has a specific format. Figure 3.4 shows this format.
You must use single quotes around the character strings. Double quotes will not work. Spacing
and capitalization are optional, except inside the single quotes. Inside them, the text is taken
literally, so any capitalization will be stored in the database exactly as you specify. If you type too
many quotes, you might reach a point where your backslash commands do not work anymore, and
your prompt will appear as testâ&#x20AC;&#x2122;>. Notice the single quote before the greater than symbol. Just

type another single quote to get out of this mode, use \r to clear the query buffer, and start again.
Notice that the 19 does not have quotes. It does not need them because the column is a numeric
column, not a character column. When you do your INSERT operations, be sure to match each piece
of data to the receiving column. Figure 3.5 shows the additional INSERT commands needed to make
the friend table match the three friends shown in Table 3.1.

3.4 Viewing Data with SELECT
You have just seen how to store data in the database. Now, let’s retrieve that data. Surprisingly,
only one command is provided to get data out of the database—SELECT. You have already used
SELECT in your first database query (see Figure 2.2 on page 7). We will now use it to show the
rows in the table friend. As shown in Figure 3.6, the entire query appears on one line. As queries
become longer, breaking them into multiple lines helps make things clearer.
Let’s look at this example in detail. First, we have the word SELECT, followed by an asterisk (*),
the word FROM, our table name friend, and a semicolon to execute the query. The SELECT starts
our command, telling the database server what is coming next. The * tells the server we want all
the columns from the table. The FROM friend indicates which table we want to see. Thus, we have
said we want all (*) columns from our table friend. Indeed, that is what is displayed—the same
data as shown in Table 3.1 on page 12.
SELECT has a large number of variations, and we will look at a few of them now. Suppose you
want to retrieve only one of the columns from the friend table. You might already suspect that the
asterisk (*) must be changed in the query. If you replace it with one of the column names, you
will see only that column. Try SELECT city FROM friend. You can choose any of the columns. You
can even choose multiple columns, by separating the names with a comma. For example, to see
first and last names only, use SELECT firstname, lastname FROM friend. Try a few more SELECT
commands until you become comfortable.
If you specify a name that is not a valid column name, you will get an error message like ERROR:

attribute ’mycolname’ not found. If you try selecting from a table that does not exist, you will
get an error message like ERROR: Relation ’mytablename’ does not exist. POSTGRESQL uses the
formal relational database terms relation and attribute in these error messages.

3.5

Selecting Specific Rows with WHERE

Let’s take the next step in controlling the output of SELECT. In the previous section, we showed
how to select only certain columns from the table. Now, we will show how to select only certain
rows. This operation requires a WHERE clause. Without a WHERE clause, every row is returned.
The WHERE clause goes immediately after the FROM clause. In the WHERE clause, you specify
the rows you want returned, as shown in Figure 3.7. The query returns the rows that have an age
column equal to 23. Figure 3.8 shows a more complex example that returns two rows.
You can combine the column and row restrictions in a single query, allowing you to select any
single cell, or a block of cells. See Figures 3.9 and 3.10.
Up to this point, we have made comparisons only on the age column. The age column is an
INTEGER. The tricky part about the other columns is that they are CHAR() columns, so you must
put the comparison value in single quotes. You also have to match the capitalization exactly. See
Figure 3.11. If you had compared the firstname column to ‘SAM’ or ‘sam’, it would have returned
no rows. Try a few more comparisons until you are comfortable with this operation.

We know how to add data to the database; now we will learn how to remove it. Removal is quite
simple. The DELETE command can quickly eliminate any or all rows from a table. The command
DELETE FROM friend will delete all rows from the table friend. The query DELETE FROM friend WHERE
age = 19 will remove only those rows that have an age column equal to 19.
Here is a good exercise. Use INSERT to insert a row into the friend table, use SELECT to verify
that the row has been properly added, then use DELETE to remove the row. This exercise combines
the ideas you learned in the previous sections. Figure 3.12 shows an example.

3.7 Modifying Data with UPDATE
How do you modify data already in the database? You could use DELETE to remove a row and then
use INSERT to insert a new row, but that is quite inefficient. The UPDATE command allows you to
update data already in the database. It follows a format similar to the previous commands.
Continuing with our friend table, suppose Mike had a birthday, so we want to update his age in
the table. The example in Figure 3.13 shows the word UPDATE, the table name friend, followed by
SET, then the column name, the equals sign (=), and the new value. The WHERE clause controls
which rows are affected by the UPDATE, just as in a DELETE operation. Without a WHERE clause, all
rows are updated.
Notice that the Mike row has moved to the end of the list. The next section will explain how
to control the order of the display.

3.8 Sorting Data with ORDER BY
In a SELECT query, rows are displayed in an undetermined order. To guarantee that the rows will
be returned from SELECT in a specific order, you must add the ORDER BY clause to the end of the
SELECT. Figure 3.14 shows the use of ORDER BY. You can reverse the order by adding DESC, as
shown in Figure 3.15. If the query also used a WHERE clause, the ORDER BY would appear after the
WHERE clause, as in Figure 3.16.
You can ORDER BY more than one column by specifying multiple column names or labels,
separated by commas. The command would then sort by the first column specified. For rows with
equal values in the first column, it would sort based on the second column specified. Of course,
this approach is not useful in the friend example because all column values are unique.

3.9

Destroying Tables

This chapter would not be complete without showing you how to remove tables. This task is
accomplished using the DROP TABLE command. For example, the command DROP TABLE friend will

remove the friend table. Both the table structure and the data contained in the table will be erased.
We will use the friend table in the next chapter, so you should not remove the table at this time.
Remember—to remove only the data in the table without removing the table structure itself, use
DELETE.

3.10

Summary

This chapter has shown the basic operations of any database:
• Table creation ( CREATE TABLE)
• Table destruction (DROP TABLE)
• Displaying (SELECT)
• Adding (INSERT)
• Replacing (UPDATE)
• Removing (DELETE)
This chapter has shown these commands in their simplest forms; real-world queries are much
more complex. The next chapters will show how these simple commands can be used to handle
some very complicated tasks.

Chapter 4

Customizing Queries
This chapter will illustrate additional capabilities of the basic SQL commands.

4.1

Data Types

Table 4.1 lists the most common column data types. Figure 4.1 shows queries using these types.
Notice that numbers do not require quotes, but character strings, dates, and times do require them.
The final SELECT uses psqlâ&#x20AC;&#x2122;s \x display mode.1 Without \x, the SELECT would have displayed
too much information to fit on one line. The fields would have wrapped around the edge of the
display, making it difficult to read. The columns would still line up, but there would be other data
in the way. Of course, another solution to field wrapping is to select fewer columns. Remember,
you can select any columns from the table in any order.
Section 9.2 covers column types in more detail.
1 See

Suppose you want to insert the name O’Donnell. You might be tempted to enter it in psql as
’O’Donnell’, but this approach will not work. The presence of a single quote inside a single-quoted
string generates a parser error. One way to place a single quote inside a single-quoted string is to
use two quotes together—for example, ’O’ ’Donnell’.2 Two single quotes inside a single-quoted
string causes one single quote to be generated. Another option is to use a backslash—for example,
’O\’Donnell’. The backslash escapes the single quote character.

4.3

Using NULL Values

Let’s return to the INSERT statement described in Section 3.3 on page 14. We will continue to
use the friend table from the previous chapter. In Figure 3.4, we specified a value for each friend
column. Suppose now that we want to insert a new row, but do not want to supply data for all
columns. That is, we want to insert information about Mark, but we do not know Mark’s age.
Figure 4.2 shows this scenario. After the table name, column names appear in parentheses.
These columns will be assigned, in order, to the supplied data values. If we were supplying data
for all columns, we would not need to name them. In this example, however, we must name the
columns. The table has five columns, but we are supplying only four data values.
The column we did not assign was age. The interesting question is, “What is in the age cell for
Mark?” The answer is that the age cell contains a NULL value.
NULL is a special value that is valid in any column. You use it when a valid entry for a field is
not known or not applicable. In the previous example, we wanted to add Mark to the database but
did not know his age. It is difficult to imagine what numeric value could be used for Mark’s age
column. Zero or -1 would be strange age values. Thus, NULL is the appropriate value for his age
column.
Suppose we have a spouse column. What value should be used if someone is not married?
A NULL value would be the proper value. For a wedding_anniversary column, unmarried people
would have a NULL value in that field. NULL values are very useful. Before databases supported
NULL values, users would put special values in columns, such as -1 for unknown numbers and
1/1/1900 for unknown dates. NULL values offer a more consistent way to mark such values.
2 That

is not a double quote between the O and D, but rather two single quotes.

CHAPTER 4. CUSTOMIZING QUERIES

26

NULL values exhibit special behavior in comparisons. Look at Figure 4.3. First, notice that
the age column for Mark is empty. It is really a NULL. In the next query, because NULL values are
unknown, the NULL row does not appear in the output. The third query often confuses people. 3
Why doesn’t the Mark row appear? The age is NULL or unknown, meaning that the database does
not know if it equals 99—and does not guess. It refuses to print it. In fact, no comparison exists
that will produce the NULL row, except the last query shown.
The tests IS NULL and IS NOT NULL are designed specifically to test for the existence of NULL
values. If you are making comparisons on columns that might contain NULL values, you must test
for them specifically.
Figure 4.4 shows an example of such a comparison. We have inserted Jack, but the city and
state were not known, so they are set to NULL. The next query’s WHERE comparison is contrived,
but illustrative. Because city and state are both NULL, you might suspect that the Jack row would
be returned. However, because NULL means unknown, we have no way to know whether the two
NULL values are equal. Again, POSTGRESQL does not guess and does not print the result.
One other issue with NULLs needs clarification. In character columns, a NULL is not the same
as a zero-length value. The empty string ’’ and NULL are different. Figure 4.5 shows an example
highlighting this difference. There are no valid numeric and date blank values, but a character
string can be blank. When viewed in psql, any blank numeric field must contain a NULL because
no blank number exists. However, there are blank strings, so blank strings and NULL values are
displayed in the same way in psql. Of course, they are not the same, so be careful not to confuse
the meaning of NULL values in character fields.

4.4

Controlling DEFAULT Values

As we learned in the previous section, columns not specified in an INSERT statement are given
NULL values. You can change this assignment by using the DEFAULT keyword. When creating a
table, the keyword DEFAULT and a value can be used next to each column type. The value will
then be used anytime the column value is not supplied in an INSERT. If no DEFAULT is defined, a
NULL is used for the column. Figure 4.6 shows a typical use of default values. The default for the
timestamp column is actually a call to an internal POSTGRESQL variable that returns the current
date and time.

4.5

Column Labels

You might have noticed the text that appears at the top of each column in the SELECT output—the
column label. The label usually is the name of the selected column. However, you can control
the text that appears at the top of each column by using the AS keyword. For example, Figure 4.7
replaces the default column label firstname with the column label buddy. You might have noticed
3 The <>

test=> -- a single line comment
test=> /* a multiline
test*>
comment */

Figure 4.9: Comment styles

that the query in Figure 2.3 on page 7 has the column label ?column?. The database server returns
this label when there is no suitable label. In that case, the result of an addition does not have an
appropriate label. Figure 4.8 shows the same query with an appropriate label added using AS.

4.6

Comments

POSTGRESQL allows you to place any text into psql for use as a comment. Two comment styles
are possible. The presence of two dashes (- -) marks all text to the end of the line as a comment.
POSTGRESQL also understands C-style comments, where the comment begins with slash-asterisk
(/*) and ends with asterisk-slash (*/). Figure 4.9 shows both comment styles. Notice how the
multiline comment is marked by a psql command prompt of *>. It is a reminder that you are in
a multiline comment, just as -> is a reminder that you are in a multiline statement, and â&#x20AC;&#x2122;> is a
reminder that you are in a multiline quoted string.

4.7

AND/OR Usage

Until now, we have used only simple WHERE clause tests. In the following sections, we will
demonstrate how to perform more complex WHERE clause testing.
Complex WHERE clause tests are done by connecting simple tests using the words AND and
OR. For illustration, new people have been inserted into the friend table, as shown in Figure 4.10.
Selecting certain rows from the table will require more complex WHERE conditions. For example,
if we wanted to select Sandy Gleason by name, it would be impossible using only one comparison

in the WHERE clause. If we tested for firstname = ’Sandy’, we would select both Sandy Gleason
and Sandy Weber. If we tested for lastname = ’Gleason’, we would get both Sandy Gleason and her
brother Dick Gleason. The proper approach is to use AND to test both firstname and lastname. This
query is shown in Figure 4.11. The AND combines the two needed comparisons.
A similar comparison could be used to select friends living in Cedar Creek, Maryland. Other
friends could live in Cedar Creek, Ohio, so the comparison city = ’Cedar Creek’ is not enough.
The proper test is city = ’Cedar Creek’ AND state = ’MD’.
Another complex test would be to select people who live in the state of New Jersey (NJ) or
Pennsylvania (PA). Such a comparison requires the use of OR. The test state = ’NJ’ OR state =
’PA’ would return the desired rows, as shown in Figure 4.12.
An unlimited number of AND and OR clauses can be linked together to perform complex
comparisons. When ANDs are linked with other ANDs, there is no possibility for confusion. The
same is true of ORs. On the other hand, when ANDs and ORs are both used in the same query, the
results can be confusing. Figure 4.13 shows such a case. You might suspect that it would return
rows with firstname equal to Victor and state equal to PA or NJ. In fact, the query returns rows
with firstname equal to Victor and state equal to PA, or state equal to NJ. In this case, the AND is

evaluated first, then the OR. When mixing ANDs and ORs, it is best to collect the ANDs and ORs
into common groups using parentheses. Figure 4.14 shows the proper way to enter this query.
Without parentheses, it is very difficult to understand a query with mixed ANDs and ORs.

4.8 Range of Values
Suppose we want to see all friends with ages between 22 and 25. Figure 4.15 shows two queries
that produce this result. The first query uses AND to perform two comparisons that both must be
true. We used <= and >= so the age comparisons included the limiting ages of 22 and 25. If we used
< and >, the ages 22 and 25 would not have been included in the output. The second query uses
BETWEEN to generate the same comparison. BETWEEN comparisons include the limiting values in
the result.

CHAPTER 4. CUSTOMIZING QUERIES

34

Comparison
less than
less than or equal
equal
greater than or equal
greater than
not equal

Comparison
begins with D
contains a D
has D in second position
begins with D and contains e
begins with D, contains e, then f
begins with non-D

Operation
LIKE ’D%’
LIKE ’%D%’
LIKE ’_D%’
LIKE ’D%e%’
LIKE ’D%e%f%’
NOT LIKE ’D%’

Table 4.3: LIKE comparisons

4.9 LIKE Comparison
Greater than and less than comparisons are possible using the operators shown in Table 4.2. Even
more complex comparisons can be made. For instance, users often need to compare character
strings to see if they match a certain pattern. Sometimes they want only fields that begin with
a certain letter or that contain a certain word. The LIKE keyword allows such comparisons. The
query in Figure 4.16 returns rows where the firstname begins with D. The percent symbol (%) means
that any characters can follow the D. Thus the query performs the test firstname LIKE ’D%’.
The test firstname LIKE ’%D%’ returns those rows where firstname contains D anywhere in the
field, not just at the beginning. The effect of having a % before and after a character is that the
character can appear anywhere in the string.
More complex tests can be performed with LIKE, as shown in Table 4.3. While the percent
symbol (%) matches an unlimited number of characters, the underscore (_) matches only a single
character. The underscore allows any single character to appear in that position. To test whether
a field does not match a pattern, use NOT LIKE. To test for an actual percent symbol (%), use
backslash-percent (\ %). To test for an actual underscore (_), use backslash-underscore (\ _).

start
end
any single character
set of characters
set of characters not equal
range of characters
range of characters not equal
zero or one of previous character
zero or multiple of previous characters
one or multiple of previous characters
OR operator

ˆ
$
.
[ccc]
[ˆccc]
[c-c]
[ˆc-c]
?
*
+
|

Table 4.5: Regular expression special characters

Attempting to find all character fields that end with a certain character can be difficult. For
CHAR() columns, like firstname, trailing spaces make trailing comparisons difficult with LIKE. Other
character column types do not use trailing spaces. Those can, for example, use the test colname
LIKE ’%g’ to find all rows that end with g. See Section 9.2 for complete coverage of character data
types.

4.10

Regular Expressions

Regular expressions allow more powerful comparisons than LIKE and NOT LIKE. Regular expression
comparisons are a unique feature of POSTGRESQL. They are very common in Unix, such as in the
Unix grep command.4
Table 4.4 lists the regular expression operators, and Table 4.5 lists the regular expression
special characters. Note that the caret (ˆ) has a different meaning outside and inside square
brackets ([ ]).
4 Actually,

in POSTGRESQL, regular expressions are like egrep extended regular expressions.

4.11. CASE CLAUSE

37

Test
begins with D
contains D
D in second position
begins with D and contains e
begins with D, contains e, and then f
contains A, B, C, or D
contains A or a
does not contain D
does not begin with D
begins with D, with one optional leading space
begins with D , with optional leading spaces
begins with D, with at least one leading space
ends with G, with optional trailing spaces

Although regular expressions are powerful, they can be complex to create. Table 4.6 shows
some examples, and Figure 4.17 shows selected queries using regular expressions. For a description of each query, see the comment above it.
Figure 4.18 shows two more complex regular expressions. The first query demonstrates how
to properly test for a trailing n. Because CHAR() columns contain trailing spaces to fill the column,
you must test for possible trailing spaces. (See Section 9.2 for complete coverage on character
data types.) The second query might seem surprising. Some might think that it returns rows
that do not contain an S. Actually, it returns all rows that have any character that is not an S. For
example, Sandy contains characters that are not S, such as a, n, d, and y, so that row is returned.
The test would prevent rows containing only S’s from being printed.
You can also test for the literal characters listed in Table 4.5. Use of a backslash removes any
special meaning from the character that follows it. For example, to test for a dollar sign, use \$.
To test for an asterisk, use \*. To test for a literal backslash, use two backslashes (\\).
Because regular expressions are so powerful, creating them can be challenging. Try some
queries on the friend table until you are comfortable with regular expression comparisons.

4.11

CASE Clause

Many programming languages have conditional statements, stating if condition is true then do
something, else do something else. This kind of structure allows execution of statements based on
some condition. Although SQL is not a procedural programming language, it does allow conditional

control over the data returned from a query. The WHERE clause uses comparisons to control row
selection. The CASE statement allows comparisons in column output. Figure 4.19 shows a query
using CASE to create a new output column containing adult or minor as appropriate, based on the
age field. Of course, the values adult and minor do not appear in the table friend. The CASE clause
allows the creation of those conditional strings.
Figure 4.20 shows a more complex example. It shows a query with multiple WHEN clauses.
The AS clause is used to label the column with the word distance. Although only SELECT examples
are shown, CASE can be used in UPDATE and other complicated situations. CASE allows the creation
of conditional values, which can be used for output or for further processing in the same query.

4.12

Distinct Rows

It is often desirable to return the results of a query with no duplicates. The keyword DISTINCT
prevents duplicates from being returned. Figure 4.21 shows the use of DISTINCT to prevent
duplicate states and duplicate city and state combinations. Notice that DISTINCT operates only on
the columns selected in the query. It does not compare nonselected columns when determining
uniqueness. Section 5.2 explains how counts can be generated for each of the distinct values.

Many functions and operators are available in POSTGRESQL. Function calls can take zero, one, or
more arguments and return a single value. You can list all functions and their arguments using
psql’s \df command. You can use psql’s \dd command to display comments about any specific
function or group of functions, as shown in Figure 4.22.
Operators differ from functions in the following ways:
• Operators are symbols, not names.
• Operators usually take two arguments.
• Arguments appear to the left and right of the operator symbol.
For example, + is an operator that takes one argument on the left and one on the right, and returns
the sum of the arguments. Psql’s \do command lists all POSTGRESQL operators and their arguments. Figure 4.23 shows a listing of operators and examples of their use. The standard arithmetic
operators—addition (+), subtraction (-), multiplication (*), division (/), modulo/remainder (%), and
exponentiation (ˆ)—honor the standard precedence rules. That is, exponentiation is performed
first; multiplication, division, and modulo second; and addition and subtraction last. You can use
parentheses to alter this precedence. Other operators are evaluated in a left-to-right manner,
unless parentheses are present.

4.14

SET, SHOW, and RESET

The SET command allows you to change various POSTGRESQL parameters. The changes remain in
effect for the duration of the database connection. Table 4.7 shows two common parameters that
can be controlled with SET.
The SET DATESTYLE command controls the appearance of dates when printed in psql, as seen
in Table 4.8. It controls the format (slashes, dashes, or year first) and the display of the month
first (US) or day first (European). The command SET DATESTYLE TO ’SQL,US’ would most likely
be selected by users in the United States, while Europeans might prefer SET DATESTYLE TO

test=> SHOW DATESTYLE;
NOTICE: DateStyle is ISO with US (NonEuropean) conventions
SHOW VARIABLE
test=> SET DATESTYLE TO ’SQL, EUROPEAN’;
SET VARIABLE
test=> SHOW DATESTYLE;
NOTICE: DateStyle is SQL with European conventions
SHOW VARIABLE
test=> RESET DATESTYLE;
RESET VARIABLE
test=> SHOW DATESTYLE;
NOTICE: DateStyle is ISO with US (NonEuropean) conventions
SHOW VARIABLE

Figure 4.24: SHOW and RESET examples

’POSTGRES,EUROPEAN’. The ISO datestyle and GERMAN datestyle are not affected by any of the
other options.
The TIMEZONE defaults to the time zone of the server or the PGTZ environment variable. The
psql client might be in a different time zone, so SET TIMEZONE allows this parameter to be changed
inside psql. See the SET manual page for a full list of SET options.
The SHOW command is used to display the current database session parameters. The RESET
command allows a session parameter to be reset to its default value. Figure 4.24 shows examples
of these commands.5
5 Your

site defaults may be different.

4.15. SUMMARY

4.15

47

Summary

This chapter has shown how simple commands can be enhanced using features like DISTINCT,
and complex WHERE clauses. These features give users great control over the execution
of queries. They were chosen by committees as important features that should be in all SQL
databases. Although you may never use all of the features mentioned in this chapter, many of them
will prove valuable when solving real-world problems.
CASE,

Chapter 5

SQL Aggregates
Users often need to summarize database information. Instead of seeing all rows, they want just
a count or total. This type of operation is called aggregation or gathering together. This chapter
focuses on POSTGRESQLâ&#x20AC;&#x2122;s ability to generate summarized database information using aggregates.

5.1

Aggregates

Table 5.1 lists five aggregates. COUNT operates on entire rows; the other four operate on specific
columns. Figure 5.1 shows examples of aggregate queries.
Aggregates can be combined with a WHERE clause to produce more complex results. For
example, the query SELECT AVG(age) FROM friend WHERE age >= 21 computes the average age of
people age 21 or older. This prevents Dick Gleason from being included in the average computation
because he is younger than 21. The column label defaults to the name of the aggregate. You can
use AS to change it, as described in Section 4.5.
NULL values are not processed by most aggregates, such as MAX(), SUM(), and AVG(); they are
simply ignored. However, if a column contains only NULL values, the result is NULL, not zero.
COUNT(*) is different in this respect. It does count NULL values because it looks at entire rows
Aggregate
COUNT(*)
SUM(colname)
MAX(colname)
MIN(colname)
AVG(colname)

using the asterisk(*). It does not examine individual columns like the other aggregates. To find
the COUNT of all non-NULL values in a certain column, use COUNT(colname). To find the number of
distinct values in a column, use COUNT(DISTINCT colname).
Figure 5.2 illustrates aggregate handling of NULL values. First, a single row containing a NULL
column is used to show aggregates returning NULL results. Two versions of COUNT on a NULL
column are shown. Notice that COUNT never returns a NULL value. Then, a single non-NULL row is
inserted, and the results shown. Notice the AVG() of 3 and NULL is 3, not 1.5, illustrating the NULL
value is not considered in the average computation. Psqlâ&#x20AC;&#x2122;s \da command lists all of the aggregates
supported by POSTGRESQL.

5.2

Using GROUP BY

Simple aggregates return one row as a result. It is often desirable, however, to apply an aggregate
to groups of rows. In queries using aggregates with GROUP BY, the aggregate is applied to rows
grouped by another column in the table. For example, SELECT COUNT(*) FROM friend returns the
total number of rows in the table. The query in Figure 5.3 shows the use of GROUP BY to count
the number of people in each state. With GROUP BY, the table is split up into groups by state, and
COUNT(*) is applied to each group in turn.
The second query shows the minimum, maximum, and average ages of the people in each state.
It also shows an ORDER BY operation carried out on the aggregate column. Because the column is
the fourth one in the result, you can identify it by the number 4. Using ORDER BY avg would have
worked as well.
You can GROUP BY more than one column, as shown in Figure 5.4. GROUP BY collects all NULL
values into a single group.

5.3 Using HAVING
One more aggregate capability is often overlookedâ&#x20AC;&#x201D;the HAVING clause. HAVING allows a user to
perform conditional tests on aggregate values. It is often employed in conjunction with GROUP BY.
With HAVING, you can include or exclude groups based on the aggregate value for that group. For
example, suppose you want to know all states in which you have more than one friend. Looking at
the first query in Figure 5.3, you can see exactly which states have more than one friend. HAVING
allows you to test the count column, as shown in Figure 5.5. Aggregates cannot be used in a WHERE
clause; they are valid only inside HAVING.

5.4

Query Tips

In Figures 5.3 and 5.5, the queries are spread over several lines. When a query has several
clauses, such as FROM, WHERE, and GROUP BY, it is best to place each clause on a separate

line. This convention makes queries easier to understand. Clear queries also use appropriate
capitalization.
In a test database, mistakes do not create a problem. In a live production database, however,
one incorrect query can cause great difficulty. It takes five seconds to issue an erroneous query,
and sometimes five days to recover from it. Double-check your queries before executing them.
This consideration is especially important for UPDATE, DELETE, and INSERT queries, because they
modify the database. Also, before performing an UPDATE or DELETE, do a SELECT or SELECT
COUNT(*) with the same WHERE clause. Make sure the SELECT result is reasonable before doing
the UPDATE or DELETE.

5.5

Summary

Sometimes users want less output rather than more. They want a total, count, average, maximum,
or minimum value for a column. Aggregates make this calculation possible. They aggregate data
into fewer rows and then send the result to the user.

Chapter 6

Joining Tables
This chapter discusses how to store data using multiple tables. Both multitable storage and
multitable queries are fundamental to relational databases.
We start this chapter by examining table and column references, which are important in
multitable queries. Then, we cover the advantages of splitting data into multiple tables. Next,
we introduce an example based on a mail-order company, showing table creation, insertion, and
queries using joins. Finally, we explore a variety of join types.

6.1

Table and Column References

Before dealing with joins, we must mention one important feature. Up to this point, all queries
have involved a single table. When a query involves multiple tables, column names can become
confusing. Unless you are familiar with each table, it is difficult to know which column names
belong to which tables. Sometimes two tables may use the same column name. For these reasons,
SQL allows you to fully qualify column names by preceding the column name with the table name.
Figure 6.1 shows an example of table name prefixing. In the figure, the first query has unqualified
column names. The second query is the same, but with fully qualified column names. A period
separates the table name from the column name.
The final query in Figure 6.1 shows another feature. Instead of specifying the table name, you
can create a table alias to take the place of the table name in the query. The alias name follows the
table name in the FROM clause. In this example, f is used as an alias for the friend table. While
these features are not important in single table queries, they are useful in multitable queries.

6.2

Joined Tables

In our friend example, splitting data into multiple tables makes little sense. However, in cases
where we must record information about a variety of things, multiple tables have benefits. Consider
57

a company that sells parts to customers through the mail. Its database has to record information
about many things: customers, employees, sales orders, and parts. It is obvious that a single table
cannot hold these different types of information in an organized manner. Therefore, we create
four tables: customer, employee, salesorder, and part. Unfortunately, putting information in different
tables also causes problems. How do we record which sales orders belong to which customers?
How do we record the parts for the sales orders? How do we record which employee received
the sales order? The solution is to assign unique numbers to every customer, employee, and part.
When we want to record the customer in the salesorder table, for example, we put the customerâ&#x20AC;&#x2122;s
number in the salesorder table. When we want to record which employee took the order, we put the
employeeâ&#x20AC;&#x2122;s number in the salesorder table. When we want to record which part has been ordered,
we put the part number in the salesorder table.
Breaking up the information into separate tables allows us to keep detailed information about
customers, employees, and parts. It also allows us to refer to those specific entries as many times
as needed by using a unique number. Figure 6.2 illustrates the joining of the separate tables we
will use.
People might question the choice of using separate tables. While not necessary, it is often
useful. Without a separate customer table, every piece of information about a customer would
have to be stored in the salesorder table every time a salesorder row was added. The customerâ&#x20AC;&#x2122;s
name, telephone number, address, and other information would have to be repeated. Any change
in customer information, such as a change in telephone number, would have to be performed in
all places in which that information is stored. With a customer table, the information is stored in
one place, and each salesorder points to the customer table. This approach is more efficient, and
it allows for easier administration and data maintenance. The advantages of using multiple tables
include the following:

The only time duplicate data should not be moved to a separate table is when all of the following
conditions are present:
• The time required to perform a join is prohibitive.
• Data lookup is unnecessary.
• Duplicate data require little storage space.
• Data are very unlikely to change.
The customer, employee, part, and salesorder example clearly benefits from multiple tables. The
process of distributing data across multiple tables to prevent redundancy is called data normalization.

6.3

Creating Joined Tables

Figure 6.3 shows the SQL statements needed to create the tables in our mail-order example. 1
The customer, employee, and part tables all have a column to hold their unique identification
numbers. The salesorder2 table includes columns to hold the customer, employee, and part
numbers associated with a particular sales order. For the sake of simplicity, we will assume that
each salesorder entry contains only one part number.
We have used underscore (_) to allow the use of multiple words in column names—for example,
customer_id. This is a common practice. You could enter the column as CustomerId, but POSTGRESQL converts all identifiers, such as column and table names, to lowercase; thus the actual
column name becomes customerid, which is not very clear. The only way to define nonlowercase
column and table names is to use double quotes. Double quotes preserve any capitalization you
supply. You can even have spaces in table and column names if you surround the name with double
quotes (")—for example, "customer id". If you decide to use this feature, you must put double
quotes around the table or column name every time it is referenced. Obviously, this practice can
be cumbersome.
1 In the real world, the name columns would be much longer, perhaps CHAR(60) or CHAR(180). You should base the length on
the longest name you may ever wish to store. Short names are used here so they display properly in the examples.
2 A table cannot be called order. The word order is a reserved keyword, for use in the ORDER BY clause. Reserved keywords
are not available as table or column names.

Keep in mind that all table and column names not protected by double quotes should consist
of only letters, numbers, and the underscore character. Each name must start with a letter, not
a number. Do not use punctuation, except the underscore, in your names. For example, address,
office, and zipcode9 are valid names, but 2pair and my# are not.
The example in Figure 6.3 also shows the existence of a column named customer_id in two
tables. This duplication occurs because the two columns contain the same type of number, a
customer identification number. Giving them the same name clearly shows which columns join
the tables together. If you wanted to use unique names, you could name the column salesorder_customer_id or sales_cust_id. This choice makes the column names unique, but still documents
the columns to be joined.
Figure 6.4 shows the insertion of a row into the customer, employee, and part tables. It also
shows the insertion of a row into the salesorder table, using the same customer, employee, and
part numbers to link the salesorder row to the other rows we inserted. For simplicity, we will use
only a single row per table.

6.4

Performing Joins

When data are spread across multiple tables, retrieval of that information becomes an important
issue. Figure 6.5 indicates how to find the customer name for a given order number. It uses
two queries. The first gets the customer_id for order number 14673. The returned customer
identification number of 648 then is used in the WHERE clause of the next query. That query finds
the customer name record where the customer_id equals 648. We call this two-query approach a
manual join, because the user manually took the result from the first query and placed that number
into the WHERE clause of the second query.
Fortunately, relational databases can perform this type of join automatically. Figure 6.6 shows
the same join as in Figure 6.5 but places it in a single query. This query shows all of the elements
necessary to perform the join of two tables:
• The two tables involved in the join are specified in the FROM clause.
• The two columns needed to perform the join are specified as equal in the WHERE clause.
• The salesorder table’s order number is tested in the WHERE clause.
• The customer table’s customer name is returned from the SELECT.
Internally, the database performs the join by carrying out the following operations:
• salesorder.order_id = 14673: Find that row in the salesorder table.
• salesorder.customer_id = customer.customer_id: From the row just found, get the customer_id. Find the equal customer_id in the customer table.
• customer.name: Return name from the customer table.

That is, the database performs the same steps as the manual join, but much faster.
Notice that Figure 6.6 qualifies each column name by prefixing it with the table name, as
discussed in Section 6.1. While such prefixing is optional in many cases, it is required in this
example because the column customer_id exists in both tables mentioned in the FROM clause,
customer and salesorder. Without such prefixing, the query would generate an error: ERROR: Column
’customer_id’ is ambiguous.
You can also perform the join in the opposite direction too. In the previous query, the order
number was supplied and the customer name returned. In Figure 6.7, the customer name is
supplied and the order number returned. The order of items in the FROM and WHERE clauses has
also been switched; the ordering of items is not important in these clauses.

6.5 Three- and Four-Table Joins
Figure 6.8 demonstrates a three-table join. In this example, the first printed column is the customer
name, and the second column is the employee name. Both columns are labeled name. You could
use AS to give the columns unique labels. Figure 6.9 shows a four-table join, using AS to make each
column label unique. The four-table join matches the arrows in Figure 6.2, with the arrows of the
salesorder table pointing to the other three tables.
Joins can also be performed among tables that are only indirectly related. Suppose you wish to
find employees who have taken orders for each customer. Figure 6.10 shows such a query. Notice
that this query displays just the customer and employee tables. The salesorder table is used to join
the two tables but does not appear in the result. The DISTINCT keyword is used because multiple
orders taken by the same employee for the same customer would make that employee appear
more than once, which was not desired. The second query uses an aggregate to return a count for
each unique customer/employee pair.
Until now, we have used only a single row in each table. As an exercise, add more customer,
employee, and part rows, and add salesorder rows that join to these new entries. You can use

Figure 6.4 as an example. Choose any unique identification numbers you like, then try the queries
already shown in this chapter with your new data.

6.6

Additional Join Possibilities

So far, all of our example joins have involved the salesorder table in some form. Suppose we want
to assign an employee to manage each customer account. If we add an employee_id column to
the customer table, the column could store the identification number of the employee assigned to
manage the customerâ&#x20AC;&#x2122;s account. Figure 6.11 shows how to perform a join between the customer
and employee tables. The first query finds the employee name assigned to manage customer
number 648. The second query shows the customers managed by employee 24. Notice that the
salesorder table is not involved in these queries.
Suppose you want to assign an employee to be responsible for answering detailed questions
about parts. You would then add an employee_id column to the part table, place valid employee
identifiers in the column, and perform queries similar to those in Figure 6.12. Adding columns to
existing tables is covered in Section 13.2.
In some cases, a join could be performed with the state column. For example, to check state
mailing codes for validity, a statecode table could be created with all valid state codes. 3 An application
could check the state code entered by the user and report an error if it is not in the statecode table.
Another example would be the need to print the full state name in queries. State names could be
stored in a separate table and joined when the full state name is desired. Figure 6.13 shows an
example of such a statename table. Thus we have two more uses for additional tables:
3 The

United States Postal Service has assigned a unique two-letter code to each U.S. state.

6.6. ADDITIONAL JOIN POSSIBILITIES

-- find the employee assigned to part number 153
SELECT employee.name
FROM
part, employee
WHERE part.employee_id = employee.employee_id AND
part.part_id = 153;
-- find the parts assigned to employee 24
SELECT part.name
FROM
part, employee
WHERE part.employee_id = employee.employee_id AND
employee.employee_id = 24
ORDER BY name;

• Check codes against a list of valid values—that is, allow only valid state codes
• Store code descriptions—that is, state code and state name

6.7

Choosing a Join Key

The join key is the value used to link rows between tables. For example, in Figure 6.4, 648 is the
customer key, appearing in the customer table to uniquely identify the row, and in the salesorder
table to refer to that specific customer row.
Some people might question whether an identification number is needed. Should the customer
name be used as a join key? Using it as the join key is not a good idea for several reasons:
• Numbers are less likely to be entered incorrectly.
• Two customers with the same name would be impossible to distinguish in a join.
• If the customer name changes, all references to that name would have to change.
• Numeric joins are more efficient than joins of long character strings.
• Numbers require less storage space than character strings.
In the statename table, the two-letter state code is probably a good join key for the following
reasons:
• Two-letter codes are easy for users to remember and enter.
• State codes are always unique.
• State codes do not change.
• Joins of short two-letter codes are not significantly slower than integer joins.
• Two-letter codes do not require significantly more storage space than integers.
Essentially, two choices for join keys exist: identification numbers and short character codes. If an
item is referenced repeatedly, it is best to use a short character code as a join key. You can display
this key to users and allow them to refer to customers and employees using codes. Users prefer to
identify items by short, fixed-length character codes containing numbers and letters. For example,
customers might be identified by six-character codes (FLE001), employees by their initials ( BAW),
and parts by five-character codes (E7245). Codes are easy to use and remember. In many cases,
users can choose the codes, as long as they are unique.
It is possible to allow users to enter short character codes and still use identification numbers
as join keys. Adding a code column to the table accomplishes this goal. For the customer table, a
new column called code can be added to hold the customer code. When the user enters a customer

code, the query can find the customer_id assigned to the customer code, then use that customer_id
in joins with other tables. Figure 6.14 shows a query using a customer code to find all order
numbers for that customer.
In some cases, identification numbers work well and codes are unnecessary, as in the following
cases:
• Items with short lifespans, such as order numbers
• Items without appropriate codes, such as payroll batch numbers
• Items used internally and not referenced by users
Defining codes for such values would be useless. It is better to allow the database to assign a
unique number to each item. Chapter 7 discusses database support for assigning unique identifiers.
No universal rule dictates when you should choose codes or identification numbers. U.S. states
are clearly better keyed on codes, because only 50 exist. The resulting codes are short, unique,
and well known by most users. At the other extreme, order numbers are best used without codes
because too many of them are possible and codes would be of little use.

6.8 One-to-Many Joins
Up to this point, when we joined two tables, one row in the first table matched exactly one row
in the second table, making the joins one-to-one joins. But what if more than one salesorder row
existed for a customer ID? Multiple order numbers would be printed. In such a one-to-many join,
one customer row would join to more than one salesorder row. Now, suppose no orders were made
by a customer. Even though a valid customer row would exist, if there were no salesorder row for
that customer identification number, no rows would be returned. We could call that situation a
one-to-none join. Section 8.3 covers outer joins, which allow unjoined rows to appear in the result.
Consider the example in Figure 6.15. Because the animal table’s 507 rabbit row joins to three
rows in the vegetable table, the rabbit row is duplicated three times in the output. This is a one-tomany join. There is no join for the 508 cat row in the vegetable table, so the 508 cat row does not
appear in the output. This is an example of a one-to-none join.

When joining tables, it is necessary to join each table mentioned in the FROM clause by specifying
joins in the WHERE clause. If you use a table name in the FROM clause but fail to join it in the
WHERE clause, the table is marked as unjoined. It is then paired with every row in the query result.
Figure 6.16 illustrates this effect using the tables from Figure 6.15. The SELECT does not join any
column from animal to any column in vegetable, causing every value in animal to be paired with
every value in vegetable. This result, called a Cartesian product, is usually not intended. When a
query returns many more rows than expected, look for an unjoined table in the query.

6.10

Table Aliases and Self-joins

In Section 6.1, you saw how to refer to specific tables in the FROM clause using a table alias.
Figure 6.17 shows a rewrite of the query in Figure 6.14 using aliases. A c is used as an alias for
the customer table, and an s is used as an alias for the salesorder table. Table aliases are handy in
these cases.

With table aliases, you can even join a table to itself in a self-join. In this case, the same table
is given two different alias names. Each alias then represents a different instance of the table.
This concept might seem to have questionable utility, but it can prove useful. Figure 6.18 shows
practical examples. For simplicity, results are not shown for these queries.
The first query in Figure 6.18 uses c as an alias for the customer table and c2 as another alias for
customer. It finds all customers in the same ZIP code as customer number 648. The second query
finds all customers in the same ZIP code as customer number 648. It then finds the order numbers
placed by those customers. We have restricted the c2 tableâ&#x20AC;&#x2122;s customer identification number to
be not equal to 648 because we do not want customer 648 to appear in the result. The third query
goes further, retrieving the part numbers associated with those orders.

6.11

Non-equijoins

Equijoins, the most common type of join, use equality (=) to join tables. Figure 6.19 shows our
first non-equijoin. The first query uses not equal (<>) to perform the join. It returns all customers
not in the same country as customer number 648. The second query uses less than (<) to perform

the join. Instead of finding equal values to join, it joins all rows later than a specific hire date. The
query returns all employees hired after employee number 24. The third query uses greater than
(>) in a similar way. It returns all parts that cost less than part number 153. Non-equijoins are not
used often, but certain queries require them.

6.12

Ordering Multiple Parts

Our mail-order example has a serious limitation: It allows only one part_id per salesorder. In the
real world, this restriction would not be acceptable. Now that we have covered many complex
join topics in this chapter, we are ready to create a more complete database layout that allows for
multiple parts per order.
Figure 6.20 shows a new version of the salesorder table. Notice that the part_id column has
been removed. The customer, employee, and part tables remain unchanged.
Figure 6.21 shows a new table, orderpart. This table is needed because the original salesorder
table could hold only one part number per order. Instead of having part_id in the salesorder table,
the orderpart table holds one row for each part number ordered. If five part numbers are in order
number 15398, then five rows will appear in the orderpart table with order_id equal to 15398.
We also add a quantity column. If a customer orders seven of the same part number, we put
only one row in the orderpart table, but set the quantity field equal to 7. We use DEFAULT to set the
quantity to 1 if no quantity is specified.
Notice that the orderpart table does not include a price field. Instead, the price is stored in the

part table. Whenever the price is needed, a join is performed to get the price. This choice allows
us to change a partâ&#x20AC;&#x2122;s price in one place, and all references to it will be updated automatically. 4
The table layout illustrates the master/detail use of tables. The salesorder table is the master
table, because it holds information common to each order, such as customer and employee identifiers and order date. The orderpart table is the detail table, because it contains the specific parts
making up the order. Master/detail tables are a common use of multiple tables.
Figure 6.22 shows a variety of queries using the new orderpart table. The queries demonstrate
increasing complexity. The first query already contains the order number of interest, so there is
no reason to use the salesorder table. It goes directly to the orderpart table to find the parts making
up the order, joining to the part table to obtain part descriptions. The second query does not have
the order number, only the customer_id and order_date. It must use the salesorder table to find the
order number, then join to the orderpart and part tables to get order quantities and part information.
The third query does not have the customer_id, but instead must join to the customer table to
get the customer_id for use with the other tables. Notice that each query displays an increasing
number of columns to the user. The final query computes the total cost of the order. It uses an
aggregate to SUM cost times (*) quantity for each part in the order.

6.13

Primary and Foreign Keys

A join is performed by comparing two columns, like customer.customer_id and salesorder.customer_id. The customer.customer_id is called a primary key because it is the unique (primary) identifier
for the customer table. The salesorder.customer_id is called a foreign key because it holds a key to
another (foreign) table.

6.14

Summary

This chapter dealt with techniqueâ&#x20AC;&#x201D;the technique of creating an orderly data layout using multiple
tables. Acquiring this skill takes practice. Expect to improve your first table layouts many times.
Good data layout can make your job easier. Bad data layout can turn queries into a nightmare.
As you create your first real-world tables, you will learn to identify good and bad data designs.
Continually review your table structures and refer to this chapter again for ideas. Do not be afraid
to redesign everything. Redesign is hard, but when it is done properly, queries become easier to
craft.
Relational databases excel in their ability to relate and compare data. Tables can be joined and
analyzed in ways you might never have anticipated. With good data layout and the power of SQL,
you can retrieve an unlimited amount of information from your database.

4 In our example, changing part.price would change the price on previous orders of the part, which would cause problems. In
the real world, we would need a partprice table to store the part number, price, and effective date for the price.

Numbering Rows
Unique identification numbers and short character codes allow references to specific rows in a
table. They were used extensively in Chapter 6. For example, the customer table had a customer_id column that held a unique identification number for each customer. The employee and part tables
included similar uniquely numbered columns that were important for joins to those tables.
While unique character codes must be supplied by users, unique row numbers can be generated
automatically using two methods. This chapter describes how to use these methods.

7.1 Object Identification Numbers (OIDs)
Every row in POSTGRESQL is assigned a unique, normally invisible number called an object identification number (OID). When the software is initialized with initdb, 1 a counter is created and
set to approximately seventeen-thousand.2 The counter is used to uniquely number every row.
Although databases may be created and destroyed, the counter continues to increase. It is used
by all databases, so identification numbers are always unique. No two rows in any table or in any
database will ever have the same object ID.3
You have seen object identification numbers already—they are displayed after every INSERT
statement. If you look back at Figure 3.4 on page 15, you will see the line INSERT 19053 1. INSERT is
the command that was executed, 19053 is the object identification number assigned to the inserted
row, and 1 is the number of rows inserted. A similar line appears after every INSERT statement.
Figure 6.4 on page 63 shows sequential object identification numbers assigned by consecutive
INSERT statements.
Normally, a row’s object identification number is displayed only by INSERT queries. However,
if the OID is specified by a non-INSERT query, it will be displayed, as shown in Figure 7.1. In that
1 See

Appendix B for a description of initdb.
less than this are reserved for internal use.
3 Technically, OID’s are unique among all databases sharing a common /data directory tree.
2 Values

example, the SELECT has accessed the normally invisible OID column. The OID displayed by the
INSERT and the OID displayed by the SELECT are the same.
Even though no OID column is mentioned in CREATE TABLE statements, every POSTGRESQL
table includes an invisible column called OID. This column appears only if you specifically access
it.4 The query SELECT * FROM table_name does not display the OID column. However, SELECT oid,
* FROM table_name will display it.
Object identification numbers can be used as primary and foreign key values in joins. Since
every row has a unique object ID, a separate column is not needed to hold the row’s unique number.
For example, in Chapter 6 we used a column called customer.customer_id. This column held the
customer number and uniquely identified each row. Alternatively, we could have used the row’s
object identification number as the unique number for each row, eliminating the need to create the
column customer.customer_id. In that case, customer.oid would be the unique customer number.
With this change, a similar change should be made in the salesorder table. We could rename
salesorder.customer_id to salesorder.customer_oid because the column now refers to an OID. The
column type should be changed as well. The salesorder.customer_id was defined as type INTEGER.
The new salesorder.customer_oid column would hold the OID of the customer who placed the order.
For this reason, we should change the column type from INTEGER to OID. Figure 7.2 shows a new
version of the salesorder table using each row’s OID as a join key.
A column of type OID is similar to an INTEGER column, but defining it as a type OID documents
that the column holds OID values. Do not confuse a column of type OID with a column named OID.
Every row has a column named OID, which is normally invisible. A row can have zero, one, or
more user-defined columns of type OID.
A column of type OID is not automatically assigned any special value from the database. Only
the column named OID is specially assigned during INSERT.
Also, the order_id column in the salesorder table could be eliminated. The salesorder.oid column
would then represent the unique order number.
4 Several

other invisible columns exist as well. The POSTGRESQL manuals cover their meaning and use.

This section covers three limitations of object identification numbers.

Nonsequential Numbering
The global nature of object identification assignment means most OIDs in a table are not sequential.
For example, if you insert one customer today, and another one tomorrow, the two customers will
not get sequential OIDs. In fact, their OIDs could differ by thousands because any INSERTs into other
tables between the two customer inserts would increment the object counter. If the OID is not
visible to users, this gap in numbering is not a problem. The nonsequential numbering does not
affect query processing. However, if users can see and enter these numbers, it might seem strange
that customer identification numbers are not sequential and have large gaps between them.

Nonmodifiable
An OID is assigned to every row during INSERT.
of a row.

UPDATE

cannot modify the system-generated OID

Not Backed Up by Default
During database backups, the system-generated OID of each row is normally not backed up. A flag
must be added to enable the backup of OIDs. See Section 20.5 for details.

7.3 Sequences
POSTGRESQL offers another way of uniquely numbering rowsâ&#x20AC;&#x201D;sequences. Sequences are named
counters created by users. After its creation, a sequence can be assigned to a table as a column
DEFAULT. Using sequences, unique numbers can be automatically assigned during INSERT.

CHAPTER 7. NUMBERING ROWS

82
Function
nextval(’name’)
currval(’name’)
setval(’name’, newval)

Action
Returns the next available sequence number, and updates the counter
Returns the sequence number from the previous nextval() call
Sets the sequence number counter to the specified value
Table 7.1: Sequence number access functions

The advantage of sequences is that they avoid gaps in numeric assignment, as happens with
OIDs.5 Sequences are ideal for use as user-visible identification numbers. If one customer is
created today, and another is created tomorrow, then the two customers will have sequential
numbers because no other table shares the sequence counter. 6
Sequence numbers are generally unique only within a single table. For example, if a table has
a unique row numbered 937, another table might have a row numbered 937 as well, assigned by a
different sequence counter.

7.4

Creating Sequences

Sequences are not created automatically, like OIDs. Instead, you must use the CREATE SEQUENCE
command. Three functions control the sequence counter, as shown in Table 7.1.
Figure 7.3 shows an example of sequence creation and sequence function usage. The first
command creates the sequence, then various sequence functions are called. Note that the SELECTs
do not include a FROM clause. Sequence function calls are not directly tied to any table. In the
figure:
• nextval() returns ever-increasing values.
• currval() returns the previous sequence value without incrementing.
• setval() sets the sequence counter to a new value.
Currval() returns the sequence number assigned by a prior nextval() call in the current session.
It is not affected by the nextval() calls of other users, which allows reliable retrieval of nextval()
assigned values in later queries.

7.5

Using Sequences to Number Rows

Configuring a sequence to uniquely number rows involves several steps:
5 This is not completely accurate. Gaps can occur if a query is assigned a sequence number as part of an aborted transaction.
See Section 10.2 for a description of aborted transactions.
6 Tables can be configured to share sequence counters, if desired.

1. Create the sequence.
2. Create the table, defining nextval() as the column default.
3. During the INSERT, do not supply a value for the sequenced column, or use nextval().
Figure 7.4 shows the use of a sequence for unique row numbering in the customer table. The
first statement creates a sequence counter named customer_seq. The second command creates
the customer table, and defines nextval(’customer_seq’) as the default for the customer_id column.
The first INSERT manually supplies the sequence value for the column. The nextval(’customer_seq’) function call will return the next available sequence number, and increment the sequence
counter. The second and third INSERTs allow the nextval(’customer_seq’) DEFAULT to be used for
the customer_id column. Remember, a column’s DEFAULT value is used only when a value is not
supplied by an INSERT statement. (This is covered in Section 4.4.) The SELECT shows that the
customer rows have been sequentially numbered.

An even easier way to use sequences exists. If you define a column of type SERIAL, a sequence
will be automatically created, and a proper DEFAULT will be assigned to the column. Figure 7.5
shows an example. The first NOTICE line indicates that a sequence was created for the SERIAL
column. Do not be concerned about the second NOTICE line in the figure. (Indexes are covered in
Section 11.1.)

7.7

Manually Numbering Rows

Some people wonder why OIDs and sequences are needed at all. Why can’t a database user just
find the highest number in use, add one, and use the result as the new unique row number? In
reality, OIDs and sequences are preferred for several reasons:

CHAPTER 7. NUMBERING ROWS

86
• Performance
• Concurrency
• Standardization

First, it is usually a slow process to scan all numbers currently in use to find the next available
number. Referring to a counter in a separate location is faster. Second, if one user gets the highest
number, and another user is looking for the highest number at the same time, the two users might
choose the same next-available highest number. Of course, in this case, the number would not be
unique. Such concurrency problems do not occur when using OIDs or sequences. Third, it is more
reliable to use database-supplied unique number generation than to generate unique numbers
manually.

7.8

Summary

Both OIDs and sequences allow the automatic unique numbering of rows. OIDs are always created
and numbered, while sequences require more work to configure. Both are valuable tools for
uniquely numbering rows.

Chapter 8

Combining SELECTs
So far, this book has covered topics such as regular expressions, aggregates, and joins. These
powerful SQL features allow the construction of complex queries. In some cases, however, even
these tools may prove inadequate. This chapter shows how SELECTs can be combined to create
even more powerful queries.

8.1

UNION, EXCEPT, and INTERSECT Clauses

Sometimes a single SELECT statement cannot produce the desired result. UNION, EXCEPT, and
INTERSECT allow SELECT statements to be chained together, enabling the construction of more
complex queries.
For example, suppose we want to output the friend tableâ&#x20AC;&#x2122;s firstname and lastname in the same
column. Normally, two queries would be required, one for each column.With UNION, however,
the output of two SELECTs can be combined in a single query, as shown in Figure 8.1. The query
combines two columns into a single output column.
UNION allows an unlimited number of SELECT statements to be combined to produce a single
result. Each SELECT must return the same number of columns. If the first SELECT returns two
columns, the other SELECTs must return two columns as well. The column types must also be
similar. If the first SELECT returns an INTEGER value in the first column, the other SELECTs must
return an INTEGER in their first columns, too.
With UNION, an ORDER BY clause can be used only at the end of the last SELECT. The ordering
applies to the output of the entire query. In Figure 8.1, the ORDER BY clause specifies the ordering
column by number. Instead of a number, we could use ORDER BY firstname because UNIONâ&#x20AC;&#x2122;s output
labels are the same as the column labels of the first SELECT.
As another example, suppose we have two tables that hold information about various animals.
One table holds information about aquatic animals, and the other contains data about terrestrial
animals. Two tables are used because each records information specific to one class of animal.
The aquatic_animal table holds information meaningful only for aquatic animals, like preferred
87

water temperature. The terrestrial_animal table holds information meaningful only for terrestrial
animals, like running speed. We could have included the animals in the same table, but keeping
them separate was clearer. In most cases, we will deal with the animal types separately.
Suppose we need to list all of the animals, both aquatic and terrestrial. No single SELECT can
show the animals from both tables. We cannot join the tables because no join key exists; joining is
not desired. Instead, we want rows from the terrestrial_animal table and the aquatic_animal table
output together in a single column. Figure 8.2 shows how these two tables can be combined with
UNION.
By default, UNION prevents duplicate rows from being displayed. For example, Figure 8.3 inserts
penguin into both tables, but penguin is not duplicated in the output. To preserve duplicates, you
must use UNION ALL, as shown in Figure 8.4.
You can perform more complex operations by chaining SELECTs. EXCEPT allows all rows to be
returned from the first SELECT except rows that appear in the second SELECT. Figure 8.5 shows
an EXCEPT query. Although the aquatic_animal table contains swordfish and penguin, the query in
Figure 8.5 returns only swordfish. The penguin is excluded from the output because it is returned
by the second query. While UNION adds rows to the first SELECT, EXCEPT subtracts rows from it.
INTERSECT returns only rows generated by all SELECTs. Figure 8.6 uses INTERSECT to display
only penguin. While several animals are returned by the two SELECTs, only penguin is returned by
both SELECTs.
You can link any number of SELECTs using these methods. The previous examples allowed

SELECT name
FROM aquatic_animal
UNION ALL
SELECT name
FROM terrestrial_animal;
name
-------------------------------swordfish
penguin
tiger
penguin
(4 rows)

Figure 8.4: UNION ALL with duplicates

test=>
test->
test->
test->
test->

SELECT
FROM
EXCEPT
SELECT
FROM

name
aquatic_animal

name
terrestrial_animal;
name
-------------------------------swordfish
(1 row)

Figure 8.5: EXCEPT restricts output from the first SELECT

8.2. SUBQUERIES

91

test=>
test->
test->
test->
test->

SELECT name
FROM aquatic_animal
INTERSECT
SELECT name
FROM terrestrial_animal;
name
-------------------------------penguin
(1 row)

Figure 8.6: INTERSECT returns only duplicated rows

multiple columns to occupy a single result column. Without the ability to chain SELECTs using
UNION, EXCEPT, and INTERSECT, it would be impossible to generate some of these results. SELECT
chaining can enable other sophisticated operations, such as joining a column to one table in the
first SELECT, then joining the same column to another table in the second SELECT.

8.2

Subqueries

Subqueries are similar to SELECT chaining. While SELECT chaining combines SELECTs on the same
level in a query, however, subqueries allow SELECTs to be embedded inside other queries. They
can perform several functions:
• They can take the place of a constant.
• They can take the place of a constant yet vary based on the row being processed.
• They can return a list of values for use in a comparison.
Subqueries can be quite complicated. If you have trouble understanding this section, skip over it
and return to it later.

Subqueries as Constants
A subquery, also called a subselect, can replace a constant in a query. While a constant never
changes, a subquery’s value is computed every time the query is executed.
As an example, let’s use the friend table from the previous chapters. Suppose we want to find
friends who are not in the same state as Dick Gleason. We could place his state in the query using
the constant string ’NJ’. If he moves to another state, however, we would have to change the
query. Using the state column is more reliable.

92

CHAPTER 8. COMBINING SELECTS

Figure 8.7 shows two ways to generate the correct result. One query uses a self-join to do the
comparison to Dick Gleason’s state. (Self-joins were covered in Section 6.10.) The last query uses
a subquery that returns the state as ’NJ’; this value is used by the upper query. The subquery has
taken the place of a constant. Unlike a constant, however, the value is computed every time the
query is executed.
Although we have used table aliases in the subquery for clarity, they are not required. A column
name with no table specification is automatically paired with a table in the current subquery. If no
matching table is found in the current subquery, higher parts of the query are searched for a match.
The state, firstname, and lastname in the subquery refer to the instance of the friend table in the
subquery. The same column names in the upper query automatically refer to the friend instance
in that query. If a column name matches two tables in the same subquery, an error is returned,
indicating the column is ambiguous.
Subqueries can also eliminate table joins. For example, consider the mail-order parts company
used in Figures 6.3 and 6.4 on page 61. To find the customer name for order number 14673, we
join the salesorder and customer tables, as shown in the first query in Figure 8.8. The second query
in the figure does not have a join, but instead gets the customer_id from a subquery. In general, if
a table is involved in only one join, and no columns from the table appear in the query result, the
join can be eliminated and the table moved to a subquery.
In this example, we have specified salesorder.customer_id and customer.customer_id to clearly
indicate the tables being referenced. However, this specification is not required. We could have
used only customer_id in both places. POSTGRESQL finds the first table in the same subquery or
higher that contains a matching column name.
Subqueries can be used anywhere a computed value is needed. Each has its own FROM and
WHERE clauses. It can also have its own aggregate, GROUP BY, and HAVING clauses. A subquery’s
only interaction with the upper query is the value it returns. This approach allows sophisticated
comparisons that would be difficult if the subquery’s clauses had to be combined with those of the
upper query.

Subqueries as Correlated Values
In addition to acting as constants in queries, subqueries can act as correlated values. Correlated
values vary based on the row being processed. A normal subquery is evaluated once and its value
used by the upper query. In a correlated subquery, the subquery is evaluated repeatedly for every
row processed.
For example, suppose you want to find the name of your oldest friend in each state. You can
accomplish this task with HAVING and table aliases, as shown in the first query of Figure 8.9.
Alternatively, you can execute a subquery for each row that finds the maximum age for that state.
If the maximum age equals the age of the current row, the row is output, as shown in the second
query. This query references the friend table two times, using the aliases f1 and f2. The upper
query uses f1. The subquery uses f2. The correlating specification is WHERE f1.state = f2.state,

which creates a correlated subquery because the subquery references a column from the upper
query. Such a subquery cannot be evaluated once and the same result used for all rows. Instead,
it must be evaluated for every row because the upper column value can change.

Subqueries as Lists of Values
The previous subqueries returned one row of data to the upper query. If any of the previous
subqueries returned more than one row, an error would be generated: ERROR: More than one tuple
returned by a subselect used as an expression. It is possible, however, to have subqueries return
multiple rows.
Normal comparison operators like equal and less-than expect a single value on the left and on
the right. For example, equality expects one value on the left of the equals sign (=) and one on
the right—for example, col = 3. Two special comparisons, IN and NOT IN, allow multiple values to
appear on the right side. For example, the test col IN (1,2,3,4) compares col against four values.
If col equals any of the four values, the comparison will return true and output the row. The test
col NOT IN (1,2,3,4) will return true if col does not equal any of the four values.
You can specify an unlimited number of values on the right side of an IN or NOT IN comparison.
More importantly, a subquery (instead of a constant) can be placed on the right side. It can then
return multiple rows. The subquery is evaluated, and its output used like a list of constant values.
Suppose we want all employees who took sales orders on a certain date. We could perform this
query in two ways. We could join the employee and salesorder tables, as shown in the first query of
Figure 8.10. Alternatively, we could use a subquery, as shown in the second query. In this case,
the subquery is evaluated and generates a list of values used by IN to perform the comparison.
The subquery is possible because the salesorder table is involved in a single join, and the query
does not return any columns from the salesorder table.
A NOT IN comparison returns true if a column’s value is not found. For example, suppose we
want to see all customers who have never ordered a product. That is, we need to find the customers
who have no sales orders. This task cannot be accomplished with a join. We need an anti-join,
because we want to find all customer rows that do not join to any salesorder row. Figure 8.11 shows
the relevant query. The subquery returns a list of customer_ids representing all customers who
have placed orders. The upper query returns all customer names where the customer_id does not
appear in the subquery output.

NOT IN and Subqueries with NULL Values
If a NOT IN subquery returns a NULL value, the NOT IN comparison always returns false. NOT IN
requires the upper column to be not equal to every value returned by the subquery. Because all
comparisons with NULL return false—even inequality comparisons—NOT IN returns false. NULL
comparisons were covered in Section 4.3.
We can prevent NULL values from reaching the upper query by adding IS NOT NULL to the
subquery. As an example, in Figure 8.11, if any NULL customer_id values existed, the query would

test=> SELECT name
test-> FROM customer
test-> WHERE customer_id NOT IN (
test(>
SELECT customer_id
test(>
FROM salesorder
test(>
);
name
-----(0 rows)

Figure 8.11: Customers who have no orders

98

CHAPTER 8. COMBINING SELECTS

return no rows. We can prevent this situation by adding WHERE customer_id IS NOT NULL to the
subquery. An IN subquery does not have this problem with NULLs.

Subqueries Returning Multiple Columns
Although most subqueries return a single column to the upper query, it is possible to handle
subqueries returning more than one column. For example, the test WHERE (7, 3) IN (SELECT col1,
col2 FROM subtable) returns true if the subquery returns a row with 7 in the first column and 3 in the
second column. The test WHERE (uppercol1, uppercol2) IN (SELECT col1, col2 FROM subtable)
performs equality comparisons between the upper query’s two columns and the subquery’s two
columns. Multiple columns in the upper query can then be compared with multiple columns in the
subquery. Of course, the number of values specified on the left of IN or NOT IN must be the same
as the number of columns returned by the subquery.

ANY, ALL, and EXISTS Clauses
IN and NOT IN are special cases of the more generic subquery clauses ANY, ALL, and EXISTS. ANY
will return true if the comparison operator is true for any value in the subquery. For example, the
test col = ANY(5,7,9) returns true if col equals any of the three values. ALL requires all subquery
values to compare as true, so col != ALL(5,7,9) returns true if col is not equal to all three values.
IN( ) is the same as = ANY( ), and NOT IN( ) is the same as <> ALL( ).
Normally, you can use operators like equal and greater-than only with subqueries returning one
row. With ANY and ALL, however, comparisons can be made with subqueries returning multiple
rows. They allow you to specify whether any or all of the subquery values, respectively, must
compare as true.
EXISTS returns true if the subquery returns any rows, and NOT EXISTS returns true if the
subquery returns no rows. By using a correlated subquery, EXISTS permits complex comparisons of
upper-query values inside the subquery. For example, two upper-query variables can be compared
in the subquery’s WHERE clause. EXISTS and NOT EXISTS do not specify anything in the upper
query, so it does not matter which columns are returned by the subquery.
Figure 8.12 shows the IN subquery from Figure 8.10, with the query rewritten using ANY and
EXISTS. Notice that the EXISTS subquery uses a correlated subquery to join the employee_id columns
of the two tables. Figure 8.13 shows the NOT IN query from Figure 8.11, with the query rewritten
using ALL and NOT EXISTS.

Summary
A subquery can represent a fixed value, a correlated value, or a list of values. You can use any
number of subqueries. You can also nest subqueries inside other subqueries.
In some cases, subqueries simply provide an alternative way to phrase a query. In others, a
subquery is the only way to produce the desired result.

8.2. SUBQUERIES

99

SELECT name
FROM
employee
WHERE employee_id IN (
SELECT employee_id
FROM
salesorder
WHERE order_date = ’7/19/1994’
);
SELECT name
FROM
employee
WHERE employee_id = ANY (
SELECT employee_id
FROM
salesorder
WHERE order_date = ’7/19/1994’
);
SELECT name
FROM
employee
WHERE EXISTS (
SELECT employee_id
FROM
salesorder
WHERE salesorder.employee_id = employee.employee_id AND
order_date = ’7/19/1994’
);

Figure 8.12: IN query rewritten using ANY and EXISTS

CHAPTER 8. COMBINING SELECTS

100

SELECT name
FROM
customer
WHERE customer_id NOT IN (
SELECT customer_id
FROM salesorder
);
SELECT name
FROM
customer
WHERE customer_id <> ALL (
SELECT customer_id
FROM salesorder
);
SELECT name
FROM
customer
WHERE NOT EXISTS (
SELECT customer_id
FROM salesorder
WHERE salesorder.customer_id = customer.customer_id
);

Figure 8.13: NOT IN query rewritten using ALL and EXISTS

8.3. OUTER JOINS

101

SELECT name, order_id
FROM
customer, salesorder
WHERE customer.customer_id = salesorder.customer_id
UNION ALL
SELECT name, NULL
FROM
customer
WHERE customer.customer_id NOT IN (SELECT customer_id FROM salesorder)
ORDER BY name;

Figure 8.14: Simulating outer joins

8.3

Outer Joins

An outer join is similar to a normal join, except that it performs special handling to prevent
unjoined rows from being suppressed in the result. For example, in the join customer.customer_id
= salesorder.customer_id, only customers who have sales orders appear in the result. If a customer
has no sales orders, he or she is suppressed from the output. If the salesorder table is used in an
outer join, however, the result will include all customers. The customer and salesorder tables will
then be joined and output, as well as one row for every unjoined customer. In the query result, any
reference to salesorder columns for these unjoined customers will return NULL.
POSTGRESQL 7.0 does not support outer joins. You can simulate them using subqueries and
UNION ALL, as shown in Figure 8.14. In this example, the first SELECT performs a normal join of
the customer and salesorder tables. The second SELECT displays customers who have no orders,
with NULL appearing as their order number.

8.4

Subqueries in Non-SELECT Queries

Subqueries can also be used in UPDATE and DELETE statements. Figure 8.15 shows two examples.
The first query deletes all customers with no sales orders. The second query sets the ship_date
equal to â&#x20AC;&#x2122;11/16/96â&#x20AC;&#x2122; for all orders made by Fleer Gearworks, Inc. The numbers after DELETE and
UPDATE indicate the number of rows affected by the queries.

8.5 UPDATE with FROM
UPDATE can include an optional FROM clause, which permits joins to other tables. The FROM clause
also allows the use of columns from other tables in the SET clause. With this capability, columns
can be updated with data from other tables.

Suppose we want to update the salesorder table’s order_date column. Some orders have order_dates earlier than the hire_date of the employee who recorded the sale. For these rows, we wish
to set the order_date equal to the employee’s hire_date. Figure 8.16 shows this query.
The FROM clause allows the use of the employee table in the WHERE and SET clauses. While
UPDATE can use subqueries to control which rows are updated, the FROM clause allows you to
include columns from other tables in the SET clause.
Actually, the FROM clause is not even required. The UPDATE in Figure 8.16 will work in the same
way without its FROM clause. POSTGRESQL automatically creates a reference to any table used in
a query. That is, the query SELECT salesorder.* automatically adds salesorder to the FROM clause
and executes the query. Likewise, the query DELETE FROM salesorder WHERE salesorder.order_date = employee.hire_date AND employee.employee_id = 24 uses the employee table. This feature
is particularly useful with DELETE because it does not support a FROM clause as SELECT and UPDATE
do.

Up to this point, all of our INSERT statements have inserted a single row. Each INSERT contained
a VALUES clause listing the constants to be inserted. Another form of the INSERT statement also
exists; it allows the output of a SELECT to be used to insert values into a table.
Suppose we wish to add all of our friends from the friend table to the customer table. As shown
in Figure 8.17, instead of a VALUES clause, INSERT can use the output of SELECT to insert data into
the table. Each column of the SELECT matches a receiving column in the INSERT. Column names
and character string constants can be used in the SELECT output. In the line INSERT 0 6, six rows
are inserted into the customer table. A zero object identifier is returned because more than one
row is inserted.
Inserting into the customer name column presents an interesting challenge. The friend table
stores first and last names in separate columns. In contrast, the customer table has only a single
name column. The solution is to combine the firstname and lastname columns, with a space
separating them. For example, a firstname of ’Dean’ and a lastname of ’Yeager’ must be inserted
into customer.name as ’Dean Yeager’. This combination becomes possible with trim() and the ||
operator. The trim() function removes trailing spaces. The two pipe symbols, ||, allow character
strings to be joined together to form a single string, in a process called concatenation. In this
example, trim(firstname), space (’ ’), and lastname are joined using ||.

8.7

Creating Tables Using SELECT

In addition to inserting into existing tables, SELECT can use an INTO clause to create a table and
place all of its output into the new table. For example, suppose we want to create a new table
called newfriend that is just like our friend table but lacks an age column. This task is easily done
with the query shown in Figure 8.18. The SELECT…INTO query performs three operations:
1. It creates a table called newfriend.
2. It uses SELECT’s column labels to name the columns of the new table.
3. It uses SELECT’s column types as the column types of the new table.

SELECT…INTO essentially combines CREATE TABLE and SELECT in a single statement. The AS
clause can be used to change the column labels and thus control the column names in the new
table. The other commands in the figure show the new table’s structure and contents.
SELECT…INTO tablename can also be written as CREATE TABLE tablename AS SELECT…. The
preceding query can then be rewritten as CREATE TABLE newfriend AS SELECT firstname, lastname,
city, state FROM friend.

8.8

Summary

This chapter has described how to combine queries in ways you probably never anticipated. It
showed how queries could be chained and placed inside other queries. In addition, it demonstrated
how UPDATE can use FROM, and how SELECT can create tables.
Although these features may seem confusing, they are very powerful. In most cases, you will
need only the simplest of these features. However, you may get that rare request that requires
one of the more complicated queries covered in this chapter. If you recognize such a query, return
to this chapter to refresh your memory.

Chapter 9

Data Types
Data types have been used in previous chapters. This chapter covers them in detail.

9.1

Purpose of Data Types

It is tempting to think that databases would be easier to use if only one data type existed—a type
that could hold any type of information, such as numbers, character strings, or dates. Although
a single data type would certainly make table creation simpler, having different data types offers
definite advantages:
Consistent Results Columns of a uniform type produce consistent results. Displaying, sorting,
aggregates, and joins deliver consistent results. No conflict arises over how different types
are compared or displayed. For example, selecting from an INTEGER column always yields
INTEGER values.
Data Validation Columns of a uniform type accept only properly formated data; invalid data are
rejected. For example, a column of type INTEGER will reject a DATE value.
Compact Storage Columns of a uniform type are stored more compactly.
Performance Columns of a uniform type are processed more quickly.
For these reasons, each column in a relational database can hold only one type of data. You cannot
mix data types within a column.
This limitation can cause some difficulties. For example, our friend table includes an age column
of type INTEGER. Only whole numbers can be placed in that column. The values “I will ask for his
age soon” or “She will not tell me her age” cannot be placed in that column. NULL can represent
“I do not know her age.” The solution is to create an age_comments column of type CHAR() to hold
comments that cannot be placed in the age field.
107

POSTGRESQL supports a large number of data types, as shown in Table 9.1. Except for the number
types, all entered values must be surrounded by single quotes.

Character String
Character string types are the most commonly used data types. They can hold any sequence
of letters, digits, punctuation, and other valid characters. Typical character strings are names,
descriptions, and mailing addresses. You can store any value in a character string. Nevertheless,
this type should be used only when other data types are inappropriate, as other types provide
better data validation, more compact storage, and better performance.
Three character string data types exist: TEXT, VARCHAR(length), and CHAR(length). TEXT does
not limit the number of characters stored. VARCHAR(length) limits the length of the field to

9.2. INSTALLED TYPES

109

length characters. Both TEXT and VARCHAR() store only the number of characters in the string.
CHAR(length) is similar to VARCHAR(), except it always stores exactly length characters. This type
pads the value with trailing spaces to achieve the specified length, and provides slightly faster
access than TEXT or VARCHAR().
Understanding why character string types differ from other data types can be difficult. For
example, you can store 763 as a character string. In that case, you will store the symbols 7, 6, and
3, not the numeric value 763. Consequently, you cannot add a number to the character string 763,
because it does not make sense to add a number to three symbols. Similarly, the character string
3/8/1992 consists of eight symbols starting with 3 and ending with 2. If you store this value in a
character string data type, it is not a date. You cannot sort the string with other values and expect
them to be in chronological order. The string 1/4/1998 is less than 3/8/1992 when both are sorted
as character strings because 1 is less than 3.
These examples illustrate why the other data types are valuable. The other types use predefined
formats for their data, and they support more appropriate operations on the stored information.
Nevertheless, there is nothing wrong with storing numbers or dates in character strings when
appropriate. The street address 100 Maple Avenue is best stored in a character string type, even
though a number is part of the street address. It makes no sense to store the street number in a
separate INTEGER field. Also, part numbers such as G8223-9 must be stored in character strings
because of the G and dash. In fact, part numbers that are always five digits, such as 32911 or
00413, should be stored in character strings as well. They are not real numbers, but symbols.
Leading zeros cannot be displayed by INTEGER fields, but are easily displayed in character strings.

Number
Number types allow the storage of numbers. The number types are INTEGER, INT2, INT8, OID,
NUMERIC(), FLOAT, and FLOAT4.
INTEGER, INT2, and INT8 store whole numbers of various ranges. Larger ranges require more
storage. For example, INT8 requires twice the storage of INTEGER and is slower that INTEGER.
OID is used to store POSTGRESQL object identifiers. Although you could use INTEGER for this
purpose, OID better documents the meaning of the value stored in the column.
NUMERIC(precision, decimal) allows user-defined digits of precision, rounded to decimal places.
This type is slower than the other number types.
FLOAT and FLOAT4 allow storage of floating-point values. Numbers are stored using 15 (FLOAT)
or 6 (FLOAT4) digits of precision. The location of the decimal point is stored separately, so large
values such as 4.78145e+32 can be represented. FLOAT and FLOAT4 are fast and have compact
storage, but can produce imprecise rounding during computations. When you require complete
accuracy of floating-point values, use NUMERIC() instead. For example, store monetary amounts
as NUMERIC().

Description
(x,y) coordinates
start and stop points of a line segment
( ) is a closed path, [ ] is an open path
opposite corner points of a rectangle
center point and radius
points form closed polygon

Table 9.2: Geometric types

Temporal
Temporal types allow storage of date, time, and time interval information. Although these data can
be stored in character strings, it is better to use temporal types, for the reasons outlined earlier in
this chapter.
The four temporal types are DATE, TIME, TIMESTAMP, and INTERVAL. DATE allows storage of
a single date consisting of a year, month, and day. The format used to input and display dates
is controlled by the DATESTYLE setting (see Section 4.14 on page 43). TIME allows storage of an
hour, minute, and second, separated by colons. TIMESTAMP stores both the date and the time—for
example, 2000-7-12 17:34:29. INTERVAL represents an interval of time, like 5 hours or 7 days.
INTERVAL values are often generated by subtracting two TIMESTAMP values to find the elapsed
time. For example, 1996–12–15 19:00:40 minus 1996–12–8 14:00:10 results in an INTERVAL value
of 7 05:00:30, which is 7 days, 5 hours, and 30 seconds. Temporal types can also handle time zone
designations.

Logical
The only logical type is BOOLEAN. A BOOLEAN field can store only true or false, and of course NULL.
You can input true as true, t, yes, y, or 1. False can be input as false, f, no, n, or 0. Although true and
false can be input in a variety of ways, true is always output as t and false as f.

Geometric
The geometric types support storage of geometric primitives. They include POINT, LSEG, PATH,
BOX, CIRCLE, and POLYGON. Table 9.2 shows the geometric types and typical values for each.

Network
The network types are INET, CIDR, and MACADDR. INET allows storage of an IP address, with or
without a netmask. A typical INET value with a netmask is 172.20.90.150 255.255.255.0. CIDR

9.3. TYPE CONVERSION USING CAST

111

stores IP network addresses. It allows a subnet mask to specify the size of the network segment.
A typical CIDR value is 172.20.90.150/24. MACADDR stores MAC (Media Access Control) addresses,
which are assigned to Ethernet network cards at the time of their manufacture. A typical MACADDR
value is 0:50:4:1d:f6:db.

Internal
A variety of types are used internally. Psql’s \dT command shows all data types.

9.3

Type Conversion Using CAST

In most cases, values of one type are converted to another type automatically. In those rare
circumstances where you need to explicitly convert one type to another, you can use CAST to
perform the conversion. To convert val to an INTEGER, use CAST(val AS INTEGER). To convert a
column date_col of type DATE to type TEXT, use CAST(date_col AS TEXT). You can also perform type
casting using double colons—that is, date_col::text or num_val::numeric(10,2).

9.4

Support Functions

Functions enable you to access specialized routines from SQL. They take one or more arguments
and return a result.
Suppose you want to uppercase a value or column. No command will perform this operation,
but a function can handle it. POSTGRESQL has a function called upper that takes a single string
argument and returns the argument in uppercase. The function call upper(col) calls the function
upper with col as its argument and returns it in uppercase. Figure 9.1 shows an example of the use
of the upper function.
POSTGRESQL provides many functions. Table 9.3 shows the most common ones, organized by
the data types supported. Psql’s \df shows all defined functions and their arguments. Section 16.1
describes all the psql commands.
If you call a function with a type for which it is not defined, you will get an error message, as
shown in the first query of Figure 9.2. In the first query, 5/8/1971 is a character string, not a date.
The second query converts 5/8/1971 to a date, so date_part() can be used.

9.5 Support Operators
Operators are similar to functions (see Section 4.13 on page 43). Table 9.4 lists the most common
operators. Psql’s \do command shows all defined operators and their arguments.

Returns
length of col
length of col, same as length()
length of col, including multibyte overhead
col with leading and trailing spaces removed
same as trim()
col with leading spaces removed
col with trailing spaces removed
col with leading and trailing str removed
col padded on the right to len characters
col padded on the right using str
col padded on the left to len characters
col padded on the left using str
col uppercased
col lowercased
col with the first letter capitalized
position of str in col
same as strpos()
col starting at position pos
same as substr()
col starting at position pos for length len
same as substr()
col with from changed to to
convert col to NUMERIC() based on mask
convert col to DATE based on mask
convert col to TIMESTAMP based on mask
round to an integer
NUMERIC() col rounded to len decimal places
truncate to an integer
NUMERIC() col truncated to len decimal places
absolute value
factorial
square root
cube root
exponential
natural logarithm
base-10 logarithm
convert col to a string based on mask
units part of col
same as date_part()
col rounded to units
BOOLEAN indicating whether col is a valid date
TIMESTAMP representing current date and time
string showing date/time in Unix format
BOOLEAN indicating whether col’s overlap in time
convert col to string based on mask
see psql’s \df for a list of geometric functions
broadcast address of col
host address of col
netmask of col
mask length of col
network address of col
return NULL if col1 equals col2, else return col1
return first non-NULL argument

Returns
append col2 on to the end of col1
BOOLEAN, col matches regular expression pattern
BOOLEAN, col does not match regular expression pattern
same as ˜, but case-insensitive
same as !˜, but case-insensitive
BOOLEAN, col matches LIKE pattern
same as ˜˜
BOOLEAN, col does not match LIKE pattern
same as !˜˜
factorial
addition
subtraction
multiplication
division
remainder/modulo
col1 raised to the power of col2
addition of temporal values
subtraction of temporal values
BOOLEAN indicating cols overlap in time

<<
<<=
>>
>>=

col1 << col2
col1 <<= col2
col1 >> col2
col1 >>= col2

BOOLEAN indicating

LIKE

!˜˜
NOT LIKE

Number

Temporal

Geometric
Network

see psql’s \do for a list of geometric operators
if col1 is a subnet of col2
BOOLEAN indicating if col1 is equal or a subnet of col2
BOOLEAN indicating if col1 is a supernet of col2
BOOLEAN indicating if col1 is equal or a supernet of col2

Meaning
current date
current time
current date and time
user connected to the database

Table 9.5: Common variables

All data types support the standard comparison operators <, <=, =, >=, >, and <>. Not all
operator/type combinations are defined, however. For example, if you try to add two DATE values,
you will get an error, as shown in the first query of Figure 9.3.

9.6 Support Variables
Several variables are defined in POSTGRESQL. These variables are shown in Table 9.5.

Arrays allow a column to store several simple data values. You can store one-dimensional arrays,
two-dimensional arrays, or arrays with any number of dimensions.
You create an array column in the same way as an ordinary column, except that you use brackets
to specify the dimensions of the array. The number of dimensions and size of each dimension are
for documentation purposes only. Values that do not match the dimensions specified at the time of
column creation are not rejected.
Figure 9.4 creates a table with one-, two-, and three-dimensional INTEGER columns. The first
and last columns have sizes specified. The first column is a one-dimensional array, also called a list
or vector. Values inserted into that column have an appearance like {3,10,9,32,24} or {20,8,9,1,4}.
That is, each value is a list of integers, surrounded by curly braces. The second column, col2, is
a two-dimensional array. Typical values for this column are {{2,9,3},{4,3,5}} or {{18,6},{32,5}}.
Notice the double braces. The outer brace surrounds two one-dimensional arrays. You can think
of this structure as a matrix, with the first one-dimensional array representing the first row of the
array, and the second representing the second row of the array. Commas separate the individual
elements as well as each pair of braces. The third column of the array_test table is a threedimensional array, holding values like {{{3,1},{1,9}},{{4,5},{8,2}}}. This three-dimensional
matrix is made up of two 2 2 matrices. Arrays of any size can be constructed.
Figure 9.5 shows a query inserting values into array_test plus several queries selecting data
from this table. Brackets are used to access individual array elements.
Any data type can be used as an array. If you need to frequently access or update individual
elements of the array, use separate columns or tables rather than arrays.

9.8

Large Objects (BLOBs)

POSTGRESQL cannot store values of more than several thousand bytes using the data types
discussed so far, nor can binary data be easily entered within single quotes. Instead, large
objectsâ&#x20AC;&#x201D;also called Binary Large Objects or BLOBSâ&#x20AC;&#x201D;are used to store very large values and
binary data.

Large objects permit storage of any operating system file, including images or large text files,
directly into the database. You load the file into the database using lo_import(), and retrieve it from
the database using lo_export().
Figure 9.6 shows an example that stores a fruit name and image. The lo_import() function
stores /usr/images/peach.jpg into the database. The function call returns an OID that is used to
refer to the imported large object. This value is stored in fruit.image. The lo_export() function uses
the OID value to find the large object stored in the database, then places the image into the new file
/tmp/outimage.jpg. The 1 returned by lo_export() indicates a successful export. The lo_unlink()
function removes large objects.
Full path names must be used with large objects because the database server runs in a different
directory than the psql client. Files are imported and exported by the postgres user, so postgres
must have permission to read the file for lo_import() and directory write permission for lo_export().
Because large objects use the local filesystem, users connecting over a network cannot use lo_import or lo_export(). They can, however, use psql’s \lo_import and \lo_export commands.

9.9. SUMMARY

9.9

119

Summary

Use care when choosing your data types. The many data types provide great flexibility. Wise
decisions about column names and types will give your database structure and consistency. The
appropriate choice also improves performance and allows efficient data storage. Do not choose
types hastilyâ&#x20AC;&#x201D;you will regret it later.

Chapter 10

Transactions and Locks
Up to this point, we have used POSTGRESQL as a sophisticated filing cabinet. However, a database
is much more. It allows users to view and modify information simultaneously. It helps ensure data
integrity. This chapter explores these database capabilities.

10.1

Transactions

Although you may not have heard the term transaction before, you have already used transactions.
Every SQL query is executed in a transaction. Transactions give databases an all-or-nothing
capability when making modifications.
For example, suppose the query UPDATE trans_test SET col = 3 is in the process of modifying
700 rows. After it has modified 200 rows, the user presses control-C or hits the computer reset
button. When the user looks at trans_test, he will see that none of the rows has been updated.
This result might surprise you. Because 200 of the 700 rows had already updated, you might
suspect that 200 rows would show as modified. However, POSTGRESQL uses transactions to
guarantee that queries are either fully completed or have no effect.
This feature is valuable. Suppose you were executing a query to add $500 to everyone’s salary
and accidentally kicked the power cord out of the wall during the update procedure. Without
transactions, the query may have updated half the salaries, but not the rest. It would be difficult to
know where the UPDATE stopped. You would wonder, “Which rows were updated, and which ones
were not?” You cannot simply re-execute the query, because some people would have already
received their $500 increase. With transactions, you can check to see if any of the rows were
updated. If one was updated, then all were updated. If not, you can simply re-execute the query.

121

CHAPTER 10. TRANSACTIONS AND LOCKS

122

test=> INSERT INTO trans_test VALUES (1);
INSERT 130057 1

Figure 10.1: INSERT with no explicit transaction

test=>
BEGIN
test=>
INSERT
test=>
COMMIT

BEGIN WORK;
INSERT INTO trans_test VALUES (1);
130058 1
COMMIT WORK;

Figure 10.2: INSERT using an explicit transaction

10.2

Multistatement Transactions

By default, each SQL query runs in its own transaction. Consider Figures 10.1 and 10.2, which
show two identical queries. Figure 10.1 is a typical INSERT query. Before POSTGRESQL starts
the INSERT, it begins a transaction. It performs the INSERT, then commits the transaction. This
step occurs automatically for any query with no explicit transaction. In Figure 10.2, the INSERT
uses an explicit transaction. BEGIN WORK starts the transaction, and COMMIT WORK commits the
transaction. The only difference between the two queries is that an implied BEGIN WORKâ&#x20AC;ŚCOMMIT
WORK surrounds the first INSERT.
Even more valuable is the ability to bind multiple queries into a single transaction. In such a
case, either all queries execute to completion or none has any effect. As an example, Figure 10.3
shows two INSERTs in a transaction. PostgreSQL guarantees that either both INSERTs succeed or
neither.
As a more complicated example, suppose you have a table of bank account balances, and
you wish to transfer $100 from one account to another account. This operation is performed
using two queries: an UPDATE to subtract $100 from one account, and an UPDATE to add $100 to
another account. The UPDATEs should either both complete or have no effect. If the first UPDATE
completes but not the second, the $100 would disappear from the bank records. It would have
been subtracted from one account, but never added to the other account. Such errors are very
hard to find. Multistatement transactions prevent them from happening. Figure 10.4 shows the
two queries bound into a single transaction. The transaction forces POSTGRESQL to perform the
queries as a single operation.
When you begin a transaction with BEGIN WORK, you do not have to commit it using COMMIT

Instead, you can close the transaction with ROLLBACK WORK and the transaction will be
discarded. The database is left as though the transaction had never been executed. In Figure 10.5,
the current transaction is rolled back, causing the DELETE to have no effect. Likewise, if any query
inside a multistatement transaction cannot be executed due to an error, the entire transaction is
automatically rolled back.

10.3

Visibility of Committed Transactions

Although we have focused on the all-or-nothing nature of transactions, they have other important
benefits. Only committed transactions are visible to users. Although the current user sees his
changes, other users do not see them until the transaction is committed.
For example, Table 10.1 shows two users issuing queries using the default mode in which
every statement is in its own transaction. Table 10.2 shows the same query with user 1 using a

multiquery transaction. User 1 sees the changes made by his transaction. User 2, however, does
not see the changes until user 1 commits the transaction.
This shielding is another advantage of transactions. They insulate users from seeing uncommitted transactions, so that users never see a partially committed view of the database.
As another example, consider the bank account query where we transferred $100 from one
bank account to another. Suppose we were calculating the total amount of money in all bank
accounts at the same time the $100 was being transferred. If we did not see a consistent view
of the database, we might see the $100 removed from the account, but not the $100 added. Our
bank account total would then be wrong. A consistent database view means that either we see the
$100 in its original account or we see it in its new account. Without this feature, we would have
to ensure that no one was making bank account transfers while we were calculating the amount of
money in all accounts.
Although this case is a contrived example, real-world database users INSERT, UPDATE, and
DELETE data all at the same time, even as others SELECT data. This activity is orchestrated by the
database so that each user can operate in a secure manner, knowing that other users will not affect
their results in an unpredictable way.

10.4

Read Committed and Serializable Isolation Levels

The previous section illustrated that users see only committed transactions. It did not address
what happens if someone commits a transaction while you are in your own transaction. In some
cases, you need to control whether other transaction commits are seen by your transaction.
POSTGRESQLâ&#x20AC;&#x2122;s default isolation level, READ COMMITTED, allows you to see other transaction
commits while your transaction is open. Figure 10.6 illustrates this effect. First, the transaction
does a SELECT COUNT(*). Then, while you are sitting at a psql prompt, someone INSERTs into the
table. The next SELECT COUNT(*) shows the newly INSERTED row. When another user commits a
transaction, it is seen by the current transaction, even if it is committed after the current transaction
started.

You can, however, prevent your transaction from seeing changes made to the database. SET
TRANSACTION ISOLATION LEVEL SERIALIZABLE changes the isolation level of the current transaction.
SERIALIZABLE isolation prevents the current transaction from seeing commits made by other
transactions. Thus, any commit made after the start of the first query of the transaction is not
visible. Figure 10.7 shows an example of a SERIALIZABLE transaction.
SERIALIZABLE isolation provides a stable view of the database for SELECT transactions. For
transactions containing UPDATE and DELETE queries, SERIALIZABLE mode is more complicated.
SERIALIZABLE isolation forces the database to execute all transactions as though they were run
serially (one after another), even if they are run concurrently. If two concurrent transactions
attempt to update the same row, serializability is impossible. In such a case, POSTGRESQL forces
one transaction to roll back.
For SELECT-only transactions, use the SERIALIZABLE isolation level when you do not want to
see other transaction commits during your transaction. For UPDATE and DELETE transactions,
SERIALIZABLE isolation prevents concurrent modification of the same data row; it should therefore
be used with caution.

Exclusive locks, also called write locks, prevent other users from modifying a row or an entire table.
Rows modified by UPDATE and DELETE are then exclusively locked automatically for the duration
of the transaction. This approach prevents other users from changing the row until the transaction
is either committed or rolled back.
Table 10.3 shows two simultaneous UPDATE transactions affecting the same row. The first
transaction must wait to see whether the second transaction commits or rolls back. If SERIALIZABLE
isolation level had been used, transaction 2 would have been rolled back automatically if transaction
1 committed.
The only time when users must wait for other users is when they are trying to modify the same
row. If they modify different rows, no waiting is necessary. SELECT queries never have to wait.
The database performs locking automatically. In certain cases, however, locking must be
controlled manually. As an example, Figure 10.8 shows a query that first SELECTs a row, then
performs an UPDATE. The problem arises because another user can modify the James row between
the SELECT and UPDATE. To prevent this problem, you can use SERIALIZABLE isolation. In this
mode, however, one of the UPDATEs would fail.
A better solution is to use SELECT…FOR UPDATE to lock the selected rows. Figure 10.9 shows
the same query using SELECT…FOR UPDATE. Another user cannot modify the James row between
the SELECT…FOR UPDATE and UPDATE. In fact, the row remains locked until the transaction ends.
You can also manually control locking by using the LOCK command. It allows specification of a
transaction’s lock type and scope. See the LOCK manual page for more information.

10.6

Deadlocks

It is possible to create an unrecoverable lock condition, called a deadlock. Table 10.4 illustrates how
two transactions might become deadlocked. In this example, each transaction holds a lock and is
waiting for the other transaction’s lock to be released. POSTGRESQL must roll back one transaction
because otherwise the two transactions will wait forever. Obviously, if they had acquired locks in
the same order, no deadlock would occur.

--- the SELECTed row is locked
-UPDATE lock_test
SET name = ’Jim’
WHERE name = ’James’;
1
COMMIT WORK;

Figure 10.9: SELECT…FOR UPDATE

10.7

Summary

Single-user database queries are concerned with getting the job done. Multiuser queries must be
designed to gracefully handle multiple users accessing the same data.
Multiuser interaction can be very confusing, because the database is constantly changing. In
a multiuser environment, improperly constructed queries can randomly fail when users perform
simultaneous operations. Queries cannot assume that rows from previous transactions still exist.
By learning about POSTGRESQL’S multiuser behavior, you are now prepared to create robust
queries. POSTGRESQL has the features necessary to construct reliable multiuser queries.

Chapter 11

Performance
In an ideal world, users would never need to be concerned about performance. The system would
tune itself. Unfortunately, we do not live in an ideal world. An untuned database can be thousands
of times slower than a tuned one, so it pays to take steps to improve performance. This chapter
shows you how to get the optimal performance from your database.

11.1

Indexes

When accessing a table, POSTGRESQL normally reads from the beginning of the table to the end,
looking for relevant rows. With an index, it can quickly find specific values in the index, then go
directly to matching rows. In this way, indexes allow fast retrieval of specific rows from a table.
For example, consider the query SELECT * FROM customer WHERE col = 43. Without an index,
POSTGRESQL must scan the entire table looking for rows where col equals 43. With an index on
col, POSTGRESQL can go directly to rows where col equals 43, bypassing all other rows.
For a large table, it can take minutes to check every row. Using an index, finding a specific row
takes fractions of a second.
Internally, POSTGRESQL stores data in operating system files. Each table has its own file, and
data rows are stored one after another in the file. An index is a separate file that is sorted by one
or more columns. It contains pointers into the table file, allowing rapid access to specific values in
the table.
POSTGRESQL does not create indexes automatically. Instead, users should create them for
columns frequently used in WHERE clauses.
To create an index, use the CREATE INDEX command, as shown in Figure 11.1. In this example,
customer_custid_idx is the name of the index, customer is the table being indexed, and customer_id
is the column being indexed. Although you can use any name for the index, it is good practice to use
the table and column names as part of the index nameâ&#x20AC;&#x201D;for example, customer_customer_id_idx or
i_customer_custid. This index is useful only for finding rows in customer for specific customer_ids.

It cannot help when you are accessing other columns, because indexes are sorted by a specific
column.
You can create as many indexes as you wish. Of course, an index on a seldom-used column is a
waste of disk space. Also, performance can suffer if too many indexes exist, because row changes
require an update to each index.
It is possible to create an index spanning multiple columns. Multicolumn indexes are sorted by
the first indexed column. When the first column contains several equal values, sorting continues
using the second indexed column. Multicolumn indexes are useful only on columns with many
duplicate values.
The command CREATE INDEX customer_age_gender_idx ON customer (age, gender) creates an
index that is sorted by age and, when several age rows have the same value, then sorted on gender.
This index can be used by the query SELECT * FROM customer WHERE age = 36 AND gender = ’F’
and the query SELECT * FROM customer WHERE age = 36.
The index customer_age_gender_idx is useless if you wish to find rows based only on gender,
however. The gender component of the index can be used only after the age value has been specified.
Thus, the query SELECT * FROM customer WHERE gender = ’F’ cannot use the index because it does
not place a restriction on age, which is the first part of the index.
Indexes can be useful for columns involved in joins, too. They can even be employed to speed
up some ORDER BY clauses.
To remove an index, use the DROP INDEX command. See the CREATE_INDEX and DROP_INDEX
manual pages for more information.

11.2

Unique Indexes

Unique indexes resemble ordinary indexes, except that they prevent duplicate values from occurring in the table. Figure 11.2 shows the creation of one table and a unique index. The index is
unique because of the keyword UNIQUE. The remaining queries try to insert a duplicate value, but
the unique index prevents this and displays an appropriate error message.
Sometimes unique indexes are created only to prevent duplicate values, not for performance
reasons. Multicolumn unique indexes ensure that the combination of indexed columns remains
unique. Unique indexes do allow multiple NULL values, however. Unique indexes both speed data
access and prevent duplicates.

The CLUSTER command reorders the table file to match the ordering of an index. This specialized
command is valuable when performance is critical and the indexed column has many duplicate
values.
For example, suppose the column customer.age has many duplicate values, and the query SELECT
* FROM customer WHERE age = 98 is executed. An index on age allows rapid retrieval of the row
locations from the index. If thousands of matching rows exist, however, they may be scattered in
the table file, requiring many disk accesses to retrieve them. CLUSTER reorders the table, placing
duplicate values next to each other. This speeds access for large queries accessing many duplicate
values.
CLUSTER even helps with range queries like col >= 3 AND col <= 5. The command places
these rows next to each other on disk, speeding indexed lookups.
In addition, CLUSTER can also speed ORDER BY processing. See the CLUSTER manual page for
more information.

11.4

VACUUM

When POSTGRESQL updates a row, it keeps the original copy of the row in the table file and writes
a new one. The original row, marked as expired, is used by other transactions still viewing the
database in its prior state. Deletions are similarly marked as expired, but not removed from the
table file.
The VACUUM command removes expired rows from the file. In the process, it moves rows from
the end of the table into the expired spots, thereby compacting the table file.
You should run VACUUM periodically to clean out expired rows. For tables that are heavily
modified, it is useful to run VACUUM every night in an automated manner. For tables with few

modifications, VACUUM should be run less frequently. The command exclusively locks the table
while processing.
You can run VACUUM in two ways. Using VACUUM alone vacuums all tables in the database.
Using VACUUM tablename vacuums a single table.

11.5

VACUUM ANALYZE

The VACUUM ANALYZE command resembles VACUUM, but also collects statistics about each column’s
proportion of duplicate values and the maximum and minimum values. POSTGRESQL uses this
information when deciding how to efficiently execute complex queries. You should run VACUUM
ANALYZE when a table is initially loaded and when a table’s data changes dramatically.
The VACUUM manual page shows all of the VACUUM options.

11.6

EXPLAIN

EXPLAIN causes POSTGRESQL to display how a query will be executed, rather than executing it.
As an example, Figure 11.3 shows a SELECT query preceeded by the word EXPLAIN. In the figure,
POSTGRESQL reports a sequential scan will be used on customer, meaning it will read the entire
table. The cost is an estimate of the work required to execute the query (the numbers are only
meaningful for comparison). The rows indicates the number of result rows expected. The width is
the number of bytes per row.
Figure 11.4 shows more interesting examples of EXPLAIN. The first EXPLAIN shows a SELECT
with the restriction customer_id = 55. The command reports another sequential scan, but the
restriction causes POSTGRESQL to estimate that ten rows will be returned. A VACUUM ANALYZE
command is then run, causing the next query to properly estimate that one row will be returned
instead of ten. An index is created, and the query rerun. This time, an index scan is used, allowing
POSTGRESQL to go directly to the rows where customer_id equals 55. The next EXPLAIN shows a
query with no WHERE restriction. POSTGRESQL realizes that the index is useless and performs a

sequential scan. The last query has an ORDER BY that matches an index, so POSTGRESQL uses an
index scan.
Even more complex queries can be studied using EXPLAIN, as shown in Figure 11.5. In this
example, tab1 and tab2 are joined on col1 and col2. Each table is sequentially scanned, and the
result sorted. The two results are then merge joined to produce output. It also supports hash join
and nested loop join methods. It chooses the join method it believes to be the fastest.

11.7

Summary

A variety of tools are available to speed up POSTGRESQL queries. Although their use is not
required, they can produce huge improvements in query speed. Section 20.8 outlines more steps
that database administrators can take to improve performance.

Chapter 12

Controlling Results
When a SELECT query is issued in psql, it travels to the POSTGRESQL server, is executed, and
the result then sent back to psql to be displayed. POSTGRESQL allows you to exert fine-grained
control over which rows are returned. This chapter explores the methods available to achieve this
goal.

12.1

LIMIT

The LIMIT and OFFSET clauses of SELECT allow the user to specify which rows to return. For
example, suppose customer has 1,000 rows with customer_id values ranging from 1 to 1,000.
Figure 12.1 shows queries using LIMIT and LIMIT…OFFSET. The first query sorts the table by
customer_id and uses LIMIT to return the first three rows. The second query is similar, except that
it skips to the 997th row before returning three rows.
Notice that each query uses ORDER BY. Although this clause is not required, LIMIT without
ORDER BY returns random rows from the query, which would be useless.
LIMIT improves performance by reducing the number of rows returned to the client. If an index
matches the ORDER BY, sometimes LIMIT can even produce results without executing the entire
query.

12.2

Cursors

Ordinarily, all rows generated by a SELECT are returned to the client. Cursors allow a SELECT
query to be named, and individual result rows retrieved as needed by the client.
Figure 12.2 shows an example of cursor usage. Note that cursor activity must take place inside
a transaction. To declare cursors, you use DECLARE…CURSOR FOR SELECT…. The result rows are
retrieved using FETCH. MOVE allows the user to move the cursor position. CLOSE releases all

LIMIT specifies which rows to return in the result. Cursors allow dynamic row retrieval. The
difference between LIMIT and cursors is that LIMIT specifies the rows as part of the SELECT, while
cursors allow dynamic fetching of rows. Both LIMIT and cursors offer new ways to tailor your
queries so that you obtain exactly the desired results.

Temporary tables are short-lived tablesâ&#x20AC;&#x201D;they exist only for the duration of a database session.
When a database session terminates, its temporary tables are automatically destroyed. Figure 13.1
illustrates this concept. In the figure, CREATE TEMPORARY TABLE creates a temporary table. On
psql exit, the temporary table is destroyed. Restarting psql reveals that the temporary table no
longer exists.
Temporary tables are visible only to the session that creates them; they remain invisible to
other users. In fact, several users can create temporary tables with the same name, and each user
will see only his version of the table. (See Table 13.1 for an example.) Temporary tables even
mask ordinary tables with the same name.
Temporary tables are ideal for holding intermediate data used by the current SQL session. For
example, suppose you need to do many SELECTs on the result of a complex query. An efficient
strategy is to execute the complex query once, then store the result in a temporary table.
As an example, Figure 13.2 uses SELECT â&#x20AC;Ś INTO TEMPORARY TABLE to collect all Pennsylvania
customers into a temporary table. It also creates a temporary index on the temporary table. The
User 1

When a table is created, only its owner can access it. If the owner wants others to have access,
he must change the table’s permissions using the GRANT command. Figure 13.4 shows some
examples of the use of GRANT. Available privileges are SELECT, UPDATE, DELETE, RULE, and ALL.
Access can be granted to individual users, groups, or everyone (PUBLIC). The rules for granting
access are covered in Section 13.6.
REVOKE removes permissions from a table. See the GRANT and REVOKE manual pages for more
information.

CREATE TABLE permtest (col INTEGER);
-- now only the owner can use permtest
GRANT SELECT ON permtest TO meyers;
-- now user â&#x20AC;&#x2122;meyersâ&#x20AC;&#x2122; can do SELECTs on permtest
GRANT ALL ON permtest TO PUBLIC;
-- now all users can perform all operations on permtest

Figure 13.4: Examples of the GRANT command

13.4

Inheritance

Inheritance allows the creation of a new table related to an existing table. Figure 13.5 shows the
creation of an inherited table. With inheritance, the child table receives all of the columns of its
parent, plus the additional columns it defines. In the example, child_test gets col1 from parent_test,
plus the column col2.
Inheritance also links rows in parent and child tables. If the parent table is referenced with an
asterisk suffix, rows from the parent and all children are accessed. Figure 13.6 shows insertion
into two tables related by inheritance. In the figure, parent_test accesses only the parent_test rows,
but parent_test* accesses both parent_test and child_test rows. That is, parent_test* accesses only
columns common to all tables. Because child_test.col2 is not in the parent table, it is not displayed.
Figure 13.7 shows that inherited tables can be layered on top of one another.
Consider a practical example that records information about employees and managers. The
table employee can hold information about nonmanagerial employees; the table manager can hold
information about managers. The manager table can inherit all the columns from employee and
have additional columns as well. You can then access nonmanagerial employees using employee,
managers using manager, and all employees including managers using employee*.
POSTGRESQL release 7.1 and later automatically accesses any inherited tables. An asterisk is
not needed after the table name. The keyword ONLY is used to prevent inherited table access.

fields from two tables
AS
customer.name, finance.credit_limit
finance.customer_id;

Figure 13.8: Examples of views

13.5

Views

Views are pseudo-tables. That is, they are not real tables, but nevertheless appear as ordinary
tables to SELECT. A view can represent a subset of a real table, selecting certain columns or certain
rows from an ordinary table. A view can even represent joined tables. Because views are assigned
separate permissions, you can use them to restrict table access so that users see only specific
rows or columns of a table.
Views are created using the CREATE VIEW command. Figure 13.8 shows the creation of several
views. The view customer_ohio selects only customers from Ohio. SELECTs on it will therefore
show only Ohio customers. The user sanders is then given SELECT access to the view. The
customer_address will show only address information. The customer_finance view is a join of
customer and finance, showing columns from both tables.
DROP VIEW removes a view. Because views are not ordinary tables, INSERTs, UPDATEs, and
DELETEs on views have no effect. The next section shows how rules can correct this problem.

Rules allow actions to take place when a table is accessed. In this way, they can modify the effects
of SELECT, INSERT, UPDATE, and DELETE.
Figure 13.9 shows a rule that prevents INSERTs into a table. The INSERT rule is named ruletest_insert and the action is NOTHING. NOTHING is a special rule keyword that does nothing.
Two types of rules exist. DO rules perform SQL commands in addition to the submitted query.
DO INSTEAD rules replace the user query with the rule action.
Figure 13.10 shows how rules can track table changes. In the figure, service_request holds
current service requests, and service_request_log records changes in the service_request table. The
figure also creates two DO rules on service_request. The rule service_request_update causes an
INSERT into service_request_log each time that service_request is updated. The special keyword
old is used to insert the pre-UPDATE column values into service_request_log; the keyword new
would refer to the new query values. The second rule, service_request_delete, tracks deletions
to service_request by inserting into service_request_log. To distinguish updates from deletes in
service_request_log, updates are inserted with a mod_type of ’U’ and deletes with a mod_type of ’D’.
In figure 13.10, DEFAULT was used for the user name and timestamp fields. A column’s default
value is used when an INSERT does not supply a value for the column. In this example, defaults
allow auto-assignment of these values on INSERT to service_request, and on rule INSERTs to service_request_log.
Figure 13.11 demonstrates the use of these rules. A row is inserted, updated, and deleted from
service_request. A SELECT on service_request_log shows the UPDATE rule recorded the pre-UPDATE
values, a U in mod_type, and the user, date, and time of the UPDATE. The DELETE rule follows a
similar pattern.

Although views ignore INSERT, UPDATE, and DELETE, rules can be used to properly handle them.
Figure 13.12 shows the creation of a table and a view on the table. In the figure, INSERTs into a
view are ignored, as are UPDATEs and DELETEs.
Figure 13.13 shows the creation of DO INSTEAD rules to properly handle INSERT, UPDATE, and
DELETE. This procedure involves changing INSERT, UPDATE, and DELETE queries on the view
to queries on realtable. Notice that the INSERT rule uses new to reference the new value to be
inserted. In contrast, UPDATE and DELETE use old to reference old values. Figure 13.14 shows
how the view properly handles modifications. It would be wise to add an index on col because the
rules do lookups on that column.
You can also create SELECT rules. In fact, views are implemented internally as SELECT rules.
Rules can even be applied to only certain rows. To remove them, use DROP RULE command. See
the CREATE_RULE and DROP_RULE manual pages for more information.
Creating a rule whose action performs the same command on the same table causes an infinite
loop. That is, POSTGRESQL will call the rule again and again from the rule action. For example, if
an UPDATE rule on ruletest has a rule action that also performs an UPDATE on ruletest, it will cause
an infinite loop. POSTGRESQL will detect the infinite loop and return an error.
Fortunately, POSTGRESQL also supports triggers. Triggers allow actions to be performed when
a table is modified. In this way, they can perform actions that cannot be implemented using rules.
See Section 18.4 for information on the use of triggers.

POSTGRESQL allows users to send signals to one another using LISTEN and NOTIFY. For example,
suppose a user wants to receive notification when a table is updated. He can register the table
name using the LISTEN command. If someone updates the table and then issues a NOTIFY command,
all registered listeners will be informed of the change. For more information, see the LISTEN and
NOTIFY manual pages.

13.8

Summary

This chapter has covered features that give administrators and users new capabilities in managing
database tables. Chapter 14 turns to restrictions that can be placed on table columns to improve
data management.

Chapter 14

Constraints
Constraints keep user data constrained, thereby helping to prevent invalid data from being entered
into the database. Defining a data type for a column is a constraint in itself. For example, a column
of type DATE constrains the column to valid dates.
This chapter covers a variety of constraints. We have already shown DEFAULT can be specified
at table creation. Constraints are defined at table creation in a similar way.

14.1

NOT NULL

The constraint NOT NULL prevents NULL values from appearing in a column. Figure 14.1 shows the
creation of a table with a NOT NULL constraint. Insertion of a NULL value, or an INSERT that would
set col2 to NULL, causes the INSERT to fail. As shown in the figure, an UPDATE of a NULL value also
fails.
Figure 14.2 adds a DEFAULT value for col2. This addition permits INSERTs that do not specify a
value for col2.

14.2

UNIQUE

The UNIQUE constraint prevents duplicate values from appearing in the column. It is implemented
by creating a unique index on a column. As indicated in Figure 14.3, UNIQUE prevents duplicates.
CREATE TABLE displays the name of the unique index created. The figure also shows that multiple
NULL values can be inserted into a UNIQUE column.
If a UNIQUE constraint consists of more than one column, UNIQUE cannot be used as a column
constraint. Instead, you must use a separate UNIQUE line to specify the columns that make up the
constraint. This approach creates a UNIQUE table constraint.
Figure 14.4 shows a multicolumn UNIQUE constraint. While col1 or col2 themselves may not
be unique, the constraint requires the combination of col1 and col2 to be unique. For example, in a
155

table that contains the driver’s license numbers of people in various states, two people in different
states might have the same license number, but the combination of their state and license number
should always be unique.

14.3

PRIMARY KEY

The PRIMARY KEY constraint, which marks the column that uniquely identifies each row, is a
combination of UNIQUE and NOT NULL constraints. With this type of constraint, UNIQUE prevents
duplicates, and NOT NULL prevents NULL values in the column. Figure 14.5 shows the creation of
a PRIMARY KEY column. Notice that an index is created automatically, and the column is defined as
NOT NULL.
Just as with UNIQUE, a multicolumn PRIMARY KEY constraint must be specified on a separate
line. In Figure 14.6, col1 and col2 are combined to form the primary key.
A table cannot have more than one PRIMARY KEY specification. Primary keys have special
meaning when using foreign keys, which are covered in the next section.

14.4

Foreign Key/REFERENCES

Foreign keys are more complex than primary keys. Primary keys make a column UNIQUE and NOT
NULL. Foreign keys, on the other hand, constrain data based on columns in other tables. They are
called foreign keys because the constraints are foreign—that is, outside the table.
For example, suppose a table contains customer addresses, and part of each address is a United
States two-character state code. If a table held all valid state codes, a foreign key constraint could
be created to prevent a user from entering invalid state codes.

Figure 14.7 shows the creation of a primary key/foreign key relationship. Foreign key constraints are created by using REFERENCES to refer to the primary key of another table. Foreign
keys link the tables together and prevent the insertion or updating of invalid data.
Figure 14.8 shows how foreign keys constrain column values. Here AL is a primary key value
in statename, so the INSERT is accepted. XX is not a primary key value in statename, so the INSERT
is rejected by the foreign key constraint.
Figure 14.9 shows the creation of the company tables from Figure 6.3, page 61, using primary
and foreign keys.
A variety of foreign key options are discussed next that make foreign keys even more powerful.

Modification of Primary Key Row
If a foreign key constraint references a row as its primary key, and the primary key row is updated
or deleted, then the default foreign key action is to prevent the operation. The foreign key options
ON UPDATE and ON DELETE, however, allow a different action to be taken. Figure 14.10 shows how
these options work. The new customer table’s ON UPDATE CASCADE specifies that if statename’s
primary key is updated, customer.state should be updated with the new value as well. The foreign
key ON DELETE SET NULL option specifies that if someone tries to delete a statename row that is
referenced by another table, the delete operation should set the foreign key to NULL.
The ON UPDATE and ON DELETE options can have the following actions:
UPDATEs and DELETEs to the primary key are prohibited if referenced by a foreign key
row. This is the default.

NO ACTION

UPDATEs to the primary key update all foreign key columns that reference it. DELETEs
on the primary key cause the deletion of all foreign key rows that reference it.

CASCADE

SET NULL

UPDATEs and DELETEs to the primary key row cause the foreign key to be set to NULL.

UPDATEs and DELETEs to the primary key row cause the foreign key to be set to its

DEFAULT.

Figure 14.11 illustrates the use of the CASCADE and NO ACTION rules. First, primarytest, which was
used in Figure 14.5, is created. Then a foreigntest table with ON UPDATE CASCADE and ON DELETE
NO ACTION is created. NO ACTION is the default, so ON DELETE NO ACTION was not required. Next, a
single row is inserted into each table, and an UPDATE on primarytest cascades to UPDATE foreigntest.
The primarytest row cannot be deleted unless the foreign key row is deleted first. Foreign key
actions offer you great flexibility in controlling how primary key changes affect foreign key rows.

Multicolumn Primary Keys
To specify a multicolumn primary key, it was necessary to use PRIMARY KEY on a separate line
in the CREATE TABLE statement. Multicolumn foreign keys have the same requirement. Using
primarytest2 from Figure 14.6, Figure 14.12 shows how to create a multicolumn foreign key.
FOREIGN KEY (col, â&#x20AC;Ś) must be used to label any multicolumn foreign key table constraints.

Handling

NULL

Values in the Foreign Key

A NULL value cannot reference a primary key. A single-column foreign key is either NULL or
matches a primary key. In a multicolumn foreign key, sometimes only part of a foreign key can
be NULL. The default behavior allows some columns in a multicolumn foreign key to be NULL and
others to be not NULL.

Using MATCH FULL in a multicolumn foreign key constraint requires all columns in the key to
be NULL or all columns to be not NULL. Figure 14.13 illustrates this case. First, the tables from
Figure 14.12 are used to show that the default allows one column of a foreign key to be set to NULL.
Next, the table matchtest is created with the MATCH FULL foreign key constraint option. MATCH
FULL allows all key columns to be set to NULL, but rejects the setting of only some multicolumn
key values to NULL.

Frequency of Foreign Key Checking
By default, foreign key constraints are checked at the end of each INSERT, UPDATE, and DELETE
query. Thus, if you perform a set of complex table modifications, the foreign key constraints must
remain valid at all times. For example, using the tables in Figure 14.7, if a new state is added and
then a new customer in the new state is inserted, the new state must be added to statename before
the customer is added to customer.
In some cases, it may not be possible to keep foreign key constraints valid between queries. For
example, if two tables are foreign keys for each other, it may not be possible to INSERT into one table
without having the other table row already present. A solution is to use the DEFERRABLE foreign
key option and SET CONSTRAINTS so that foreign key constraints are checked only at transaction
commit. With this approach, a multiquery transaction can make table modifications that violate
foreign key constraints inside the transaction as long as the foreign key constraints are met at

transactions commit. Figure 14.14 is a contrived example of this case; the proper way to perform
this query is to INSERT into primarytest first, then INSERT into defertest. In complex situations, such
reordering might not be possible, so DEFERRABLE and SET CONSTRAINTS should be used to defer
foreign key constraints. A foreign key may also be configured as INITIALLY DEFERRED, causing the
constraint to be checked only at transaction commit by default.
You can name constraints if desired. The constraint names will appear in constraint violation
messages and can be used by SET CONSTRAINTS. See the CREATE_TABLE and SET manual pages for
more information.

14.5

CHECK

The CHECK constraint enforces column value restrictions. Such constraints can restrict a column,
for example, to a set of values, only positive numbers, or reasonable dates. Figure 14.15 shows an
example of CHECK constraints using a modified version of the friend table from Figure 3.2, page 13.
This figure has many CHECK clauses:
state Forces the column to be two characters long. CHAR() pads the field with spaces, so state
must be trim()-ed of trailing spaces before length() is computed.
age Forces the column to hold only positive values.
gender Forces the column to hold either M or F.
last_met Forces the column to include dates between January 1, 1950, and the current date.
table Forces the table to accept only rows where firstname is not ED or lastname is not RIVERS.
The effect is to prevent Ed Rivers from being entered into the table. His name will be rejected
if it is in uppercase, lowercase, or mixed case. This restriction must be implemented as a
table-level CHECK constraint. Comparing firstname to ED at the column level would have
prevented all EDs from being entered, which was not desired. Instead, the desired restriction
is a combination of firstname and lastname.
Next, the example tries to INSERT a row that violates all CHECK constraints. Although the CHECK
failed on the friend2_last_met constraint, if that were corrected, the other constraints would prevent
the insertion. By default, CHECK allows NULL values.

14.6

Summary

This chapter covered a variety of constraints that help restrict user data within specified limits.
With small databases, constraints are of marginal benefit. With databases holding millions of rows,
however, they help keep database information organized and complete.

Importing and Exporting Data
COPY allows rapid loading and unloading of user tables. This command can write the contents of a
table to an ASCII file or load a table from an ASCII file. These files can be used for backup purposes
or to transfer data between POSTGRESQL and other applications.
The first section of this chapter describes the use of COPY to unload and load database tables.
The next part of the chapter shows how to use COPY to share data with other applications. The
final section contains tips for using COPY.

15.1

Using COPY

COPY…TO allows you to copy the contents of a table to a file. The file can later be read using
COPY…FROM.
Figure 15.1 shows the creation of a table with columns of various types. Two rows are then
inserted into copytest. SELECT shows the contents of the table, and COPY…TO writes the table to
the file /tmp/copytest.out. The rows are then deleted. Finally, COPY…FROM reloads the table, as
shown by the last SELECT.
COPY provides a quick way to load and unload tables. It is used for database backup (see
Section 20.5). The following sections cover various COPY features that are important when reading
or writing COPY files in other applications.

15.2

COPY File Format

COPY…TO can export data to be loaded into other applications, and COPY…FROM can import data
from other applications. If you are constructing a file for use with the COPY command or are reading
a COPY file in another application, it is important to understand COPY’s file format.
Figure 15.2 shows the contents of the COPY file from Figure 15.1. First, \q exits psql to an

operating system prompt. Then, the Unix cat1 command displays the file /tmp/copytest.out. This
file contains one line for every row in the table. Columns in the file are separated by tabs. These
tabs are called delimiters because they delimit (that is, separate) columns.
Tabs are difficult to see because they look like multiple spaces. The next command processes
the file using sed 2 to display tabs as <TAB>. This option clearly shows the tabs in the file, which
differ from spaces.
The columns in Figure 15.2 do not line up as they do in psql, because they are of different
lengths. The value of textcol in the first line is longer than the value in the second line. The lack of
alignment is expected because the COPY file is designed for easy processing, with one tab between
each column. It is not designed for display purposes.

15.3

DELIMITERS

You can easily change the default tab column delimiter. COPY’s USING DELIMITERS option allows
you to set the column delimiter. In Figure 15.3, setting the delimiter to a pipe symbol (|) causes
the output file to use pipes to separate columns.
If a COPY file does not use the default tab column delimiter, COPY…FROM must employ the
proper USING DELIMITERS option. As shown in Figure 15.4, if a file uses pipes rather than tabs
as column delimiters, COPY…FROM must specify pipes as delimiters. The first COPY…FROM fails
because it cannot find a tab to separate the columns. The second COPY…FROM succeeds because
the proper delimiter for the file was used.
1 Non-Unix
2 The sed

operating system users would use the type command.
operating system command replaces one string with another. See the sed(1) manual page for more information.

test=> COPY copytest FROM stdin;
Enter data to be copied followed by a newline.
End with a backslash and a period on a line by itself.
test> 3 77.43
coffee f
test> \.
test=> COPY copytest TO stdout;
1
23.99
fresh spring water
t
2
55.23
bottled soda
t
3
77.43
coffee f
test=>

Figure 15.5: COPY using stdin and stdout

15.4

COPY Without Files

COPY can also be used without files. The command can, for example, use the same input and output
locations used by psql. The special name stdin represents the psql input, and stdout represents
the psql output. Figure 15.5 shows how you can use stdin to supply COPY input directly from
your keyboard. For clarity, text typed by the user appears in bold. The gaps in the second typed
line were generated by pressing the tab key. The user typed \ . to exit COPY…FROM. A COPY to
stdout operation displays the COPY output on your screen, which can be useful when using psql in
automated scripts.

15.5

Backslashes and NULL Values

There is potential for confusion if the character used as a column delimiter also exists in user data.
If both appear the same way in the file, COPY…FROM would be unable to determine whether the
character is a delimiter or user data.
COPY avoids any confusion by specially marking delimiters appearing in user data. It precedes
them with a backslash ( \ ). If a pipe is the delimiter, COPY…TO uses pipes (|) for delimiters, and
backslash-pipes ( \ | ) for pipes in user data. In Figure 15.6, for example, each column is separated
by a pipe, but the pipe that appears in user data is output as abc \ |def.
Use of a backslash causes any character that follows it to be treated specially. As a result, a
backslash in user data is output as two backslashes ( \ \ ).
Another special backslash used in Figure 15.6 is \ N, which represents NULL. It prevents NULL
values from being confused with user values.
To change the default NULL representation, you use WITH NULL AS. For example, the command COPY copytest TO ’/tmp/copytest.out’ WITH NULL AS ’?’ will output NULL values as ques-

Description
tab if using default delimiter tab
pipe if using pipe as the delimiter
NULL if using the default NULL output
backspace
form feed
newline
carriage return
tab
vertical tab
character represented by octal number ###
backslash

Table 15.1: Backslashes understood by COPY

tion marks. Unfortunately, it will make a user column containing a single question mark indistinguishable from a NULL in the file. To output NULL values as blank columns, use the command COPY
copytest TO ’/tmp/copytest.out’ WITH NULL AS ’\,’. To treat empty columns as NULL values on
input, use COPY copytest FROM ’/tmp/copytest.out’ WITH NULL AS ’\,’.
Table 15.1 summarizes the delimiter, NULL, and backslash handling of COPY. The first two
lines in the table show that preceding a character with a backslash prevents the character from
being interpreted as a delimiter. The next line shows that \ N means NULL under the default
representation. The other backslash entries show simple representations for common characters.
The last line shows that a double-backslash is required to represent a literal backslash.

15.6. COPY TIPS

15.6

175

COPY Tips

You must use full path names with the COPY command because the database server runs in a
different directory than the psql client. Files are read and written by the postgres user, so postgres
must have permission to read the file for COPY…FROM and directory write permission for COPY…TO.
Because COPY uses the local file system, users connecting over a network cannot use file names.
They can use stdin and stdout, or psql’s \copy command.
By default, the system-generated OID column is not written out, and loaded rows receive new
OIDs. COPY…WITH OIDS allows OIDs to be written and read.
COPY writes only entire tables. To COPY only part of a table, use SELECT…INTO TEMPORARY
TABLE with an appropriate WHERE clause and then COPY the temporary table to a file.
See the COPY manual page for more detailed information.

15.7

Summary

COPY can be thought of as a crude INSERT and SELECT command. It imports and exports data in a
very generic format, which makes it ideal for use by other applications and for backup purposes.

This section summarize the capabilities of psql. The psql manual has detailed information about
each item. See Chapter 2 for an introduction to psql.

Query Buffer Commands
Table 16.1 shows the commands used to control psql’s query buffer. There is one item of particular
interest, edit (\e), which allows editing of the query buffer. The \e command loads the contents of
the query buffer into the default editor. When the user exits the editor, the editor’s contents are
reloaded into the query buffer, ready for execution. The environment variable EDITOR specifies
the default editor.

General Commands
Table 16.2 lists psql’s general commands. Psql’s local copy interface allows copy operations using
files local to the computer running psql, rather than local to the computer running the database
server. Later sections in this chapter cover the use of the \set, \unset, and \pset commands.

Output Format Options
The \pset command controls the output format used by psql. Table 16.3 lists the available formatting
commands, and Figure 16.1 shows examples of their use. In the figure, \pset tuples_only causes
psql to show only data rows, suppressing table headings and row counts. The tuples_only does not

180
Modifies
Field alignment
Field separator
One field per line
Rows only
Table title
Enable HTML
HTML table tags

Command
\a
\f
\x
\t
\C
\H
\T

Argument
separator

title
tags

Table 16.4: psql’s output format shortcuts

take a second argument, as it is an on/off parameter. The first \pset tuples_only turns it on, and the
second one turns it off. The second \pset in the figure causes psql to display NULL as (null).

Output Format Shortcuts
In addition to using \pset, some output format options have shortcuts, as shown in Table 16.4.

Variables
The \set command sets a variable, and \unset removes a variable. To access a variable you precede
its name with a colon. The \set command used alone lists all defined variables.
Figure 16.2 shows the use of psql variables. The first variable assigned, num_var, is accessed
in the SELECT query by preceding the variable name with a colon. The second \set command places
the word SELECT into a variable, then uses that variable to perform a SELECT query. The next
example uses backslash-quotes (\’) to create a string that contains single quotes. This variable
then replaces a quoted string in a query. With date_var, grave accents (`) allow a command to be run
and the result placed into a variable. In this case, the output of the Unix date command is captured
and placed into a variable. The assignment to date_var2 combines the use of backslash-quotes
and grave accents to run the date command and surround it with single quotes. The final SELECT
shows that date_var2 holds a quoted date string that can be used in queries.
Psql includes a number of predefined variables, as listed in Table 16.5. The variables in the
first group contain useful information; the others affect psql’s behavior. Some of the predefined
variables do not take an argument but are activated using \set, and deactivated using \unset.

Listing Commands
You can find a great deal of information about the current database by using psql’s listing commands,
as shown in Table 16.6. They provide information about tables, indexes, functions, and other
objects defined in the database.

Argument
name
name
name
name
name
name
name
name
name
name
name
name
name

Table 16.6: psqlâ&#x20AC;&#x2122;s listing commands

Most listing commands take an optional name parameter, which can be specified as a regular
expression. For example, \dt sec displays all table names beginning with sec, and \dt .*x.* shows
all table names containing an x. Regular expressions were covered in Section 4.10.
When you are using listing commands, the descriptions of the various data types and functions
are called comments. POSTGRESQL predefines many comments, and the COMMENT command allows
users to define their own as well. The \dd command and others then display these comments. See
the COMMENT manual page for more information.
Many commands support the use of an optional plus sign, which shows additional information.
For example, \dT lists all data types, \dT+ includes the size of each type, and \df+ shows additional
information about functions. With the other commands, a plus sign causes the comments for the
object to be displayed.

Large Object Commands
Psql has a local large object interface that supports large object operations using files local to the
computer running psql, rather than local to the computer running the database server. Table 16.7

shows the local large object commands supported.

CHAPTER 16. DATABASE QUERY TOOLS

184
Large Objects
Import
Export
Unlink
List

Command
\lo_import
\lo_export
\lo_unlink
\lo_list

Argument
file
oid file
oid

Table 16.7: psql’s large object commands

Psql Command-line Arguments and Start-up File
You can change the behavior of psql when starting a new session. Psql is normally started from
the command line by typing psql followed by the database name. However, psql accepts extra
arguments between psql and the database name, which modify psql’s behavior. For example, psql
-f file test will read commands from file, rather than from the keyboard. Table 16.8 summarizes
psql’s command-line options. Consult the psql manual page for more detailed information.
Another way to change the behavior of psql on start-up is to create a file called .psqlrc in your
home directory. Each time psql starts, it executes any backslash or SQL commands in that file.

16.2

Pgaccess

Pgaccess is a graphical database tool that is used for accessing tables, queries, views, sequences,

functions, reports, forms, scripts, users, and schemas. It is written using the POSTGRESQL TCL/TK
interface. Its source code appears in pgsql/src/bin/pgaccess.
Figure 16.3 shows the opening window of pgaccess. The tabs on the left show the items that
can be accessed. The menu at the top permits database actions, table import/export, and object
creation, deletion, and renaming.
Figure 16.4 shows the table window. This window allows table rows to be viewed and modified.
The pgaccess manual page and help screens cover its capabilities in more detail.

16.3

Summary

This chapter covered psql and pgaccess, the most popular POSTGRESQL query tools. They are
valuable tools for accessing POSTGRESQL.

Programming Interfaces
Psql is ideal for interactively entering SQL commands and for running automated scripts, but it is

not ideal for writing applications. Fortunately, POSTGRESQL has interfaces for many programming
languages. Programming languages include variables, functions, conditional evaluation, looping,
and complex input/output routines, all of which are required for writing good applications.
Table 17.1 shows the supported programming interfaces. These language interfaces allow
applications to pass queries to POSTGRESQL and receive results. The compiled languages execute
more quickly, but are more difficult to program than the interpreted ones.
This chapter will show the same application using each of the interfaces listed in Figure 17.1.
The application is a very simple one that prompts the user for a United States state code and
outputs the state name that goes with the code. Figure 17.1 shows the sample application being
run. For clarity, the text typed by the user appears in bold. The program displays a prompt, the
user types AL, and the program displays Alabama. Although state codes are unique, the application
is written to allow multiple query return values. The application uses the statename table, which
is recreated in Figure 17.2.
Additional information about POSTGRESQL interfaces is available in the Programmerâ&#x20AC;&#x2122;s Manual
mentioned in Appendix A.2.
Enter a state code:
Alabama

LIBPQ is the native C interface to POSTGRESQL. Psql and most other interfaces use it internally
for database access.
Figure 17.3 shows how LIBPQ is used. The application code communicates with the user’s
terminal and uses LIBPQ for database access. It turn, LIBPQ sends queries to the database server
and retrieves results.
Figure 17.4 shows the sample program using LIBPQ to access POSTGRESQL. The sample program performs the following tasks:
• Establish a database connection
• Prompt for and read the state code
• Form an appropriate SQL query
• Pass the SQL query to LIBPQ
• Have POSTGRESQL execute the query
• Retrieve the query results from LIBPQ
• Display the results to the user
• Terminate the database connection
All interactions with the database are accomplished via
functions are called by the sample program:

PQconnectdb() Connect to the database.
PQexec() Send the query to the database.
PQntuples() Return the number of rows (tuples) in the result.
PQgetvalue() Return a specific row and column of the result.
PQclear() Free resources used by the result.
PQfinish() Close the database connection.
These functions are the most common LIBPQ functions. The Programmer’s Manual covers all of
this interface’s functions and shows additional examples.

17.2

Pgeasy (LIBPGEASY)

LIBPGEASY is a simplified C interface that hides some of the complexity of LIBPQ. Figure 17.5
shows a LIBPGEASY version of our state code application. No error checking is required because
LIBPGEASY automatically terminates the program if an error occurs. You can change this default
using on_error_continue().

17.3

Embedded C (ECPG)

Rather than using function calls to perform SQL queries, ECPG allows SQL commands to be embedded
in a C program. The ECPG preprocessor converts lines marked by EXEC SQL to native SQL calls.
The resulting file is then compiled as a C program.
Figure 17.6 shows an ECPG version of our application. The interface implements the ANSI
embedded SQL C standard, which is supported by many database systems.

Interface include files are typically installed in /usr/local/pgsql/include. The compiler flag
-I is needed to ensure that the compiler searches the specified directory for include files—for
example, -I/usr/local/pgsql/include.
Interface libraries are typically installed in /usr/local/pgsql/lib. The compiler flag -L is
needed to ensure that the compiler searches the directory for library files—for example, L/usr/local/pgsql/lib.
The compiler flag -l is needed for the compiler to link to a specific library file. To link to libpq.a
or libpq.so, the flag -lpq is needed. Because the -l flag knows that the file begins with lib, -llibpq
is not correct—just -lpq.
The commands to compile myapp for various interfaces are listed below:
LIBPQ cc -I/usr/local/pgsql/include -o myapp myapp.c -L/usr/local/pgsql/lib -lpq
LIBPGEASY cc -I/usr/local/pgsql/include -o myapp myapp.c -L/usr/local/pgsql/lib -lpgeasy
ECPG ecpg myapp.pgc

Notice that each interface has its own library. ECPG requires the ecpg preprocessor to be run
before compilation. LIBPQ++ requires the use of a different compiler.

17.6

Assignment to Program Variables

POSTGRESQL is a network-capable database. That is, the database server and user application
can be run on different computers. Because character strings have the same representation on
all computers, they are used for communication between the user program and database server.
Queries are submitted as character strings, and results are passed back as character strings.
This approach provides reliable communication even when the two computers involved are quite
different.
The sample programs in this chapter perform SELECTs on a CHAR(30) column. Because query
results are returned as character strings, returned values can be assigned directly to program
variables. In contrast, noncharacter string columns, like INTEGER and FLOAT, cannot be assigned
directly to integer or floating-point variables. A conversion might be required instead.
For example, when you are using LIBPQ or LIBPQ++, a SELECT on an INTEGER column does
not return an integer from the database, but rather a character string that must be converted to
an integer by the application. An INTEGER is returned as the string ’983’ rather than the integer
value 983. To assign this value to an integer variable, you use the C library function atoi()—for
example, var = atoi(colval).
One exception involves BINARY cursors, which return binary representations of column values.
You can assign results from BINARY cursors directly to program variables. However, because

CHAPTER 17. PROGRAMMING INTERFACES

196

they return column values in binary format, both the application and the database server must be
running on the same computer or at least on computers with the same CPU architecture. See the
DECLARE manual page for more information on BINARY cursors.
LIBPGEASY uses fetch() to return values directly into program variables. This function should
place results into character string variables or use BINARY cursors if possible.
ECPG automatically converts data returned by POSTGRESQL to the proper format before assignment to program variables.
The interpreted languages covered later in this chapter have type-less variables, so they do
not have this problem.

17.7

ODBC

ODBC (Open Database Connectivity) is an interface used by some applications and applicationbuilding tools to access SQL databases. This middleware layer is not meant for programming
directly, but rather for communicating with other applications.
The ODBC source code is located in pgsql/src/interfaces/odbc. It can be compiled on both
Unix and non-Unix operating systems.

17.8

Java (JDBC)

Figure 17.8 shows a Java version of the same application.
The interfaceâ&#x20AC;&#x2122;s source code is located in pgsql/src/interfaces/jdbc. Once the interface is
compiled, the file postgresql.jar should be copied to the directory containing the other jar files.
The full path name of postgresql.jar must then be added to the CLASSPATH environment variable.
Java programs are compiled using javac and run using java. Java is both a compiled and
interpreted language. It is compiled for speed, but interpreted when executed so that any computer
can run the compiled program.

17.9

Scripting Languages

Up to this point, the interfaces discussed have used compiled languages. Compiled languages
require user programs to be compiled into CPU instructions.
The remaining interfaces are scripting languages. Scripting languages execute more slowly
than compiled languages, but offer several benefits:

• No compilation required
• More powerful commands
• Automatic creation of variables
• Variables that can hold any type of data

17.10

Perl

Figure 17.9 shows our state code application in Perl. Perl is a good choice for writing scripts and
small applications. It is popular for processing text files and generating dynamic Web pages using
CGI (Common Gateway Interface). A Perl/DBI interface is also available.

Figure 17.10 shows a TCL version of our application. This interface’s specialty is accessing
other toolkits and applications. The TK graphical interface toolkit is one example. TCL uses it for
graphical applications. The TK toolkit has become so popular that several other scripting languages
also use it as their graphical interface library.

17.12

Python

Python, an object-oriented scripting language, is considered to be a well-designed language, with
code that is easy to read and maintain. Figure 17.11 shows the state code application written in
Python. This interface’s source code is located in pgsql/src/interfaces/python.

for name in conn.query(
"SELECT name \
FROM statename \
WHERE code = ’"+state_code+"’").getresult():
sys.stdout.write(’%s\n’ % name)

# send the query

# print the value returned

Figure 17.11: Python sample program

17.13

PHP

PHP is used for Web browser access to POSTGRESQL. With PHP, database commands can be
embedded in Web pages.
Two Web pages are required for our state code application: one for data entry and another
for display. Figure 17.12 shows a Web page that allows entry of a state code. Figure 17.13
shows a second Web page that performs a SELECT and displays the results. Normal Web page
commands (HTML tags) begin with < and end with >. By contrast, PHP code begins with <? and
ends with ?>. The PHP interface does not ship with POSTGRESQL, but can be downloaded from
http://www.php.net .

17.14

Installing Scripting Languages

The interpreted languages described in this chapter all require a database interface to be installed
into the language. This task is done by either recompiling the language or dynamically loading the
interface into the language. The following list gives details about each interface:
Perl Use loads the POSTGRESQL interface into the Perl interpreter.
TCL/TK TCL/TK offers three interface options:

• A prebuilt TCL interpreter called pgtclsh
• A prebuilt TCL/TK interpreter called pgtksh, like TCL/TK’s wish
• A loadable library called libpgtcl
Python Import loads the POSTGRESQL interface into the Python interpreter.
PHP PHP must be recompiled to access POSTGRESQL.

17.15

Summary

All interface source code is located in pgsql/src/interfaces. Each interface includes sample source
code for use in writing your own programs.
These interfaces allow the creation of professional database applications. Each interface has
certain advantages. Some are easier, some are faster, some are more popular, and some work
better in certain environments. Choosing an interface is often a difficult task. The information in
this chapter should make that choice easier.

Functions and Triggers
Chapter 17 focused on client-side programming—programs that run on the user’s computer and interact with the POSTGRESQL database. Server-side functions, sometimes called stored procedures,
run inside the database server rather than in the client application.
Several good uses for server-side functions exist. For example, if a function is used by many
applications, it can be embedded into the database server. With this approach, each application
no longer needs a copy of the function. Whenever the function is needed, the client can simply
call it. Unlike client-side functions, server-side functions can be called within SQL queries. Also,
functions centrally installed in the server are easily modified. When a function is changed, all client
applications immediately start using the new version.
Table 9.3 on page 113 lists many preinstalled server-side functions, like upper() and date_part().
This chapter shows how to create your own functions. It also covers special server-side functions
called triggers, which are called automatically when a table is modified.

18.1

Functions

Server-side functions can be written in several languages:
•

SQL

•

PL/PGSQL

•

PL/TCL

•

PL/Perl

• C
SQL and PL/PGSQL functions are covered in this chapter. C functions are more complex and are
covered in Chapter 19.
203

SQL functions allow you to name queries and store them in the database for later access. This
section describes a variety of SQL functions of increasing complexity.
Functions are created with the CREATE FUNCTION command and removed with DROP FUNCTION.
CREATE FUNCTION requires the following information:
• Function name
• Number of function arguments
• Data type of each argument
• Function return type
• Function action
• Language used by the function action
Figure 18.1 shows the creation of a simple SQL function to convert a temperature from Fahrenheit
to centigrade degrees. It supplies the following information to CREATE FUNCTION:
• Function name is ftoc
• Function takes one argument of type float
• Function returns a float
• Function action is SELECT ($1 - 32.0) * 5.0 / 9.0;
• Function language is SQL

Although most functions return only one value, SQL functions can return multiple values using
SETOF. Function actions can also contain INSERTs, UPDATEs, and DELETEs as well as multiple
queries separated by semicolons.
The function action in ftoc() uses SELECT to perform a computation. It does not access any
tables. The $1 in the SELECT is automatically replaced by the first argument of the function call. If
a second argument were present, it would be represented as $2.
When the query SELECT ftoc(68) is executed, it calls ftoc(). This function replaces $1 with 68
and then makes the computation. In a sense, it executes a SELECT inside a SELECT. The outer
SELECT calls ftoc(), and ftoc() uses its own SELECT to perform the computation.
Constants in the function contain decimal points, so floating-point computations are performed.
Without them, division would be performed using integers. For example, the query SELECT 1/4
returns 0, whereas SELECT 1.0/4.0 returns 0.25.
Figure 18.2 shows an SQL server-side function to compute a tax. The casts to NUMERIC(8,2)
are required because the result of the computation must be rounded to two decimal places. This
function uses the more compact double-colon form of type casting, rather than CAST. Section 9.3
provides more information about type casting. SELECT tax(100) performs a simple computation,
similar to ftoc().
One powerful use of server-side functions is their use in SQL queries. Figure 18.3 shows the
use of tax() with the part table from Figure 6.3. In this figure, three rows are inserted into the table,
then a SELECT displays columns from the part table with additional computed columns showing the
tax and the cost plus tax.
Figure 18.4 shows a more complex function that computes shipping charges. This function
uses CASE to compute shipping charges based on weight. It calls shipping() to generate a detailed
analysis of the tax and shipping charges associated with each part. It prints the part number, name,
cost, tax, subtotal of cost plus tax, shipping charge, and total of cost, tax, and shipping charge. The
SELECT uses trim() to remove trailing spaces and narrow the displayed result.

If the tax rate or shipping charges change, you can easily modify the function to reflect the new
rates. Simply use DROP FUNCTION to remove the function and then recreate it with new values.
All user applications will automatically begin using the new version because the computations are
embedded in the database, not in the user applications.
Server-side functions can also access database tables. Figure 18.5 shows an SQL function that
internally accesses the statename table. It looks up the proper state name for the state code
supplied to the function.
Figure 18.6 shows two queries which yield identical results, though using different approaches.
The first query joins the customer and statename tables. The second query does a SELECT on
customer; for each row, getstatename() is then called to find the customer’s state name. The two
queries yield the same result only if each customer row joins to exactly one statename row. If any
customer rows did not join to a statename row or joined to many statename rows, the results would
be different. Also, because the second query executes the SQL function for every row in customer,
it works more slowly than the first query.

18.3

PL/PGSQL Functions

PL/PGSQL is another language intended for server-side functions. It is a true programming language.
While SQL functions allow only argument substitution, PL/PGSQL includes features such as variables,
conditional evaluation, and looping.
PL/PGSQL is not installed in each database by default. To use it in database test, you must install
it by running createlang plpgsql test from the operating system prompt.
Figure 18.7 shows a PL/PGSQL version of the SQL function getstatename from Figure 18.5. It
illustrates several PL/PGSQL features:

A special form of SELECT that allows query results to be placed into variables. It
should not be confused with SELECT * INTO.

SELECT INTO

RETURN

Exits and returns a value from the function.

Figure 18.8 shows a more complicated PL/PGSQL function. It accepts a text argument and returns
the argument in uppercase, with a space between each character. The next SELECT uses this
result to display a report heading. This function illustrates the use of variables and WHILE loops in
PL/PGSQL.
Figure 18.9 shows an even more complicated PL/PGSQL function. It takes a state name as a
parameter and finds the proper state code. Because state names are longer than state codes, they
are often misspelled. This function deals with misspellings by performing lookups in several ways.
First, it attempts to find an exact match. If that attempt fails, it searches for a unique state name
that matches the first 2, 4, or 6 characters, up to the length of the supplied string. If a unique state
is not found, the function returns an empty string ( ’ ’). Figure 18.10 shows several getstatecode()
function calls. The getstatecode() function illustrates three new PL/PGSQL features:
%TYPE Data type that matches a database column.
RECORD
FOUND

Data type that stores the result of a SELECT.

A predefined BOOLEAN variable that represents the status of the previous SELECT INTO.

Many other PL/PGSQL features are covered in the User’s Manual mentioned in Appendix A.2.
Figure 18.11 shows a PL/PGSQL function that provides a server-side interface for maintaining the
statename table. The function change_statename performs INSERT, UPDATE, and DELETE operations
on the statename table. The function is called with a state code and state name. If the state code
is not in the table, it is inserted. If it already exists, the state name is updated. If the function
is called with an empty state name ( ’ ’), the state is deleted from the table. The function returns
true (’t’) if statename was changed, and false (’f’) if statename was unmodified. Figure 18.12 shows
examples of its use.

18.4

Triggers

Rules allow SQL queries to be executed when a table is accessed (see Section 13.6). Triggers offer
an alternative way to perform actions on INSERT, UPDATE, or DELETE. They are ideal for checking
or modifying a column value before it is added to the database.
Triggers and rules are implemented differently, however. Triggers call server-side functions
for each modified row, whereas rules rewrite user queries or add queries. The former are ideal
for checking or modifying a row before it is added to the database. The latter are ideal when the
action affects other tables.

WHILE i <= len LOOP
-- test 2,4,6,... chars for match
search_str = trim(substr(state_str, 1, i)) || ’’%’’;
SELECT INTO matches COUNT(*)
FROM statename
WHERE name LIKE search_str;
IF matches.count = 0
-- no matches, failure
THEN RETURN NULL;
END IF;
IF matches.count = 1
-- exactly one match, return it
THEN
SELECT INTO statename_rec *
FROM statename
WHERE name LIKE search_str;
IF FOUND
THEN
RETURN statename_rec.code;
END IF;
END IF;
i := i + 2;
-- >1 match, try 2 more chars
END LOOP;
RETURN ’’’’ ;
END;’
LANGUAGE ’plpgsql’;

With triggers, special server-side functions can be called every time a row is modified. These
special functions can be written in any server-side language except SQL. They control the action
taken by the query. For example, they can reject certain values or modify values before they are
added to the database. Triggers that return NULL cause the operation that caused the trigger to be
ignored.
Server-side trigger functions are special because they have predefined variables to access
the row that caused the trigger. For INSERT triggers, the variable new represents the row being
inserted. For DELETE, the variable old represents the row being deleted. For UPDATE, triggers can
access the pre-UPDATE row using old and the post-UPDATE row using new. These variables are the
same as the old and new variables employed in rules.
Figure 18.13 shows the creation of a special server-side trigger function called trigger_insert_update_statename. This function uses the new RECORD variable to perform the following actions:
• Reject a state code that is not exactly two alphabetic characters
• Reject a state name that contains nonalphabetic characters
• Reject a state name less than three characters in length
• Uppercase the state code
• Capitalize the state name
If a user enters invalid data, RAISE EXCEPTION aborts the current query and displays an appropriate
error message. Validity checks can also be performed using CHECK constraints (see Section 14.5).
Uppercase and capitalization occur by simply assigning values to the new variable. The function
return type is opaque because new is returned by the function.
CREATE TRIGGER causes trigger_insert_update_statename() to be called every time a row is
inserted or updated in statename. The remaining queries in Figure 18.13 show three rejected
INSERTs as well as a successful INSERT that is properly uppercased and capitalized by the function.
Trigger functions can be quite complicated. They can perform loops, SQL queries, and any
operation supported in server-side functions. See the CREATE_TRIGGER and DROP_TRIGGER manual
pages for additional information.

18.5

Summary

Server-side functions allow programs to be embedded into the database. These programs can be
accessed from client applications and used in database queries. Moving code into the server allows
for increased efficiency, maintainability, and consistency. Triggers are special server-side functions
that are called when a table is modified.

Extending POSTGRESQL Using C
Although POSTGRESQL offers a large number of functions, operators, data types, and aggregates,
sometimes users may still need to create their own. Chapter 18 showed how to create functions
in languages other than C. This chapter covers C functions and the creation of custom operators,
data types, and aggregates that behave just like the ones already present in POSTGRESQL.
Extending POSTGRESQL in this way involves several steps:
1. Write C code to implement the new functionality.
2. Compile the C code into an object file that contains CPU instructions.
3. Issue CREATE FUNCTION commands to register the new functions.
4. Issue the proper commands if creating operators, data types, or aggregates:
•

CREATE OPERATOR

•

CREATE TYPE

•

CREATE AGGREGATE

The full details of extending POSTGRESQL are beyond the scope of this book. This chapter
will therefore provide just an overview of this topic. The Programmer’s Manual mentioned in
Appendix A.2 has more detailed information.

19.1

Write the C Code

The best way to add a new function, operator, data type, or aggregate is to start with a copy of
a file from the POSTGRESQL source directory pgsql/src/backend/utils/adt. Start with a file that
contains functions similar to the ones you need, but make sure that your new function names are
unique.
219

For example, Chapter 18 included a ftoc() SQL function that converted a temperature from
Fahrenheit to centigrade degrees. Figure 19.1 shows a C function that converts from centigrade
to Fahrenheit.
While writing C functions, you may find it necessary to execute SQL queries from inside the
function. The server programming interface (SPI) allows C functions to execute SQL queries and
process results from within these functions.

19.2

Compile the C Code

The next step is to compile the C file into an object file that contains CPU instructions. As part of
this step, you must create a special object file that can be dynamically linked into the POSTGRESQL
server. Many operating systems require special flags to create an object file that can be dynamically
linked. The best way to find the required flags is to go to pgsql/src/test/regress and type make
clean and then make regress.so.1 This command will display the compile commands used to
generate the dynamically linkable object file regress.so. The -I compile flags allow searching for
include files. Other flags are used for generating dynamic object files; use them to compile your
C code into a dynamically linkable object file. You may need to consult your operating system
documentation for assistance in locating the proper flags.

19.3

Register the New Functions

Now that you have created a dynamically linkable object file, you must register its functions with
POSTGRESQL. The CREATE FUNCTION command registers a new function by storing information in
the database. Figure 19.2 shows the CREATE FUNCTION command for ctof. The function ctof takes
1 Some

operating systems may use gmake rather than make. Also, some operating systems will use regress.o rather than

a float argument and returns a float. The SQL data type float is the same as the C type double used
in ctof(). The dynamically linkable object file is specified as /users/pgman/sample/ctof.so and is
written in the C language.
A single object file can contain many functions. You must use CREATE FUNCTION to register
each function you want to access from POSTGRESQL. CREATE FUNCTION also allows nonobject files
to be used as functions (see Chapter 18).
Once the functions are registered, they can be called just like POSTGRESQL internal functions.
Figure 19.3 shows the ctof() function used in a SELECT statement. See CREATE_FUNCTION for more
information.

19.4

Create Operators, Types, and Aggregates

Optionally, you can build operators, types, and aggregates using functions. CREATE OPERATOR,
CREATE TYPE, and CREATE AGGREGATE register that a set of functions should behave as an operator,
type, or aggregate. They name the new operator, type, or aggregate, and then call the supplied
function whenever its name is accessed. See CREATE_OPERATOR, CREATE_TYPE, and CREATE_AGGREGATE for more information.

CHAPTER 19. EXTENDING POSTGRESQL USING C

222

19.5

Summary

Extending POSTGRESQL is a complicated process. This chapter has covered only the basic concepts. Refer to the Programmerâ&#x20AC;&#x2122;s Manual for more detailed information.

Chapter 20

Administration
This chapter covers a variety of administrative tasks. The chapter assumes POSTGRESQL is
installed and running. If it is not, see Appendix B.

20.1

Files

When POSTGRESQL is installed, it creates files in its home directory, typically /usr/local/pgsql.
This directory holds all the files needed by POSTGRESQL in various subdirectories:
/bin POSTGRESQL command-line programs, such as psql.
/data Configuration files and tables shared by all databases. For example, pg_shadow is a table

shared by all databases.
/data/base A subdirectory for each database. Using the du and ls commands, administrators can

display the amount of disk space used by each database, table, or index.
/doc POSTGRESQL documentation.
/include Include files used by various programming languages.
/lib Libraries used by various programming languages. This subdirectory also contains files used
during initialization and sample configuration files that can be copied to /data and modified.
/man POSTGRESQL manual pages.

20.2

Creating Users

To create new users, you run createuser from an operating system prompt. Initially, only the
POSTGRESQL superuser, typically postgres, can create new users. Other users can be given
permission to create new users and databases.
223

POSTGRESQL user names do not have to be operating system users. For installations using
database password authentication, a createuser flag allows passwords to be assigned.
Users are removed with dropuser. The CREATE USER, ALTER USER, and DROP USER commands
are available in SQL.
POSTGRESQL also supports the creation of groups using CREATE GROUP in SQL. GRANT permissions can be specified using these groups.
Figure 20.1 shows examples of user administration commands. In the figure, one user is
created from the command line, a second user is created in psql, and a user is modified. Next, a
group is created and given table permissions. Finally, the program reconnects to the database as a
different user, which is possible because the site has local users configured with trust access. This
issue is covered in Section 20.4.
These commands can be performed only by a user with create user privileges. More information
about each command can be found in the manual pages.

To create new databases, you run createdb from an operating system prompt. Initially, only the
POSTGRESQL superuser can create new databases. Other users can be given permission to create
new databases.
The createdb program creates a new database by making a copy of the template1 database.
This database is created when POSTGRESQL is first initialized. Any modifications to template1 will
appear in subsequently created databases.
Databases are removed with dropdb. The CREATE DATABASE and DROP DATABASE commands
are also available in SQL.
Figure 20.2 shows one database created from the command line and another one created
through psql. A database is then destroyed, and a connection made to a new database. Additional
information about each command can be found in the manual pages.

20.4

Access Configuration

POSTGRESQL allows administrators to control database access. Access can be granted based on
the database, user, or TCP/IP network address. By default, POSTGRESQL allows database access
only to users logged into the computer running the database server. To enable network access,
the postmaster must be started with the -i flag.

226

CHAPTER 20. ADMINISTRATION

Database access is controlled via the data/pg_hba.conf file, which is located in the POSTGRESQL
home directory. It contains several types of configuration entries:

Local
Local entries control access by users logged into the same computer as the database server. Local
connections use Unix domain sockets. The following per-database authentication options are
available:
• trust—Trust users connecting to this database.
• password—Require a password of users connecting to this database.
• crypt—Like password, except send the password in an encrypted manner. This method is
more secure than password.
• reject—Reject all connection requests for this database.

Host and Hostssl
Host and hostssl entries control TCP/IP network access. They include host and netmask fields.
These entries support all of the local options, plus the following:
• ident—Use a remote ident server for authentication.
• krb4—Use Kerberos IV authentication.
• krb5—Use Kerberos V authentication.
These entries are effective only if the postmaster uses the -i option. Hostssl controls access via
the Secure Socket Layer (SSL) if enabled in the server.

User Mappings
By default, passwords used by password and crypt appear in the pg_shadow table. This table is
managed by createuser and ALTER USER.
In addition, password takes an optional argument that specifies a secondary password file which
overrides pg_shadow. This file contains user names and passwords of people who are allowed to
connect. Using this method, a set of users can be given access to certain databases. See the
pg_passwd manual page for more information on creating secondary password files. Currently, crypt
does not support secondary password files.
The ident entry also takes an optional argument that specifies a special map name to map ident
user names to database user names. The file data/pg_ident.conf records these mappings.

20.5. BACKUP AND RESTORE

227

Examples
Local entries are configured on per-database hosts. A database entry of all applies to all databases.
In data/pg_hba.conf, the lines
local
host

all
all

127.0.0.1

255.255.255.255

trust
trust

cause all local users to be trusted. The first line affects users connecting via Unix domain sockets;
the second line controls local users connecting to the same machine by TCP/IP. The local machine
is accessed as TCP/IP address 127.0.0.1 (localhost).
Both host and hostssl entries require the additional specification of host addresses and network
masks. The lines
host
host

all
all

192.168.34.0
192.168.90.0

255.255.255.255
255.255.255.0

crypt
password

force all users from host 192.168.34.0 and network 192.168.90.0 to provide passwords. Crypt
encrypts the passwords that are sent; password sends passwords over the network without encryption. The line
host

all

192.168.98.0

255.255.255.255

password finance

is similar to the previous entries, except that it uses the user names/passwords stored in finance
to authenticate users.
The lines
host
host

sales
sales

192.168.7.12
192.168.7.64

255.255.255.255
255.255.255.255

ident
ident support

use ident on the remote machine to verify the users connecting to database sales from the hosts
192.168.7.12 and 192.168.7.64. The second entry uses the support user name mapping in data/pg_ident.conf.
Connections from hosts and networks not appearing in the file are rejected. For more information, see the file data/pg_hba.conf and the Administratorâ&#x20AC;&#x2122;s Guide discussed in Appendix A.2.
For database client applications, the environment variables PGHOST, PGPORT, PGUSER, PGPASSWORD, PGDATESTYLE, PGTZ, PGCLIENTENCODING, and PGDATABASE are helpful for setting default
connection parameters and options. The POSTGRESQL documentation provides more information
about them.

20.5

Backup and Restore

Database backups allow databases to be restored if a disk drive fails, a table is accidentally dropped,
or a database file is accidentally deleted. If the databases are idle, a standard file system backup

will suffice as a POSTGRESQL backup. If the databases are active, you must use the pg_dumpall
utility to create a reliable backup. This utility outputs a consistent snapshot of all databases into a
file that can be included in a file system backup. In fact, once a pg_dumpall file has been created,
you do not need to back up the /data/base database files. A few configuration files in /data, such
as data/pg_hba.conf, should be included in a file system backup because they do not appear in the
pg_dumpall file. The pg_dump utility can dump a single POSTGRESQL database.
To restore from a backup using a pg_dumpall file, POSTGRESQL must be initialized, any manually
edited configuration files restored to /data, and the database dump file run by psql. This action
will recreate and reload all databases.
Individual databases can be reloaded from pg_dump files by creating a new database and loading
it using psql. For example, Figure 20.3 creates an exact copy of the test database. It dumps the
contents of the database into the file /tmp/test.dump. A new database called newtest is created,
then the dump file is loaded into the new database.
Dump files contain ordinary SQL queries and COPY commands. Because the files contain
database information, they should be created so that only authorized users have permission to
read them. See the pg_dump and pg_dumpall manual pages for more information about these
commands.

20.6

Server Start-up and Shutdown

The POSTGRESQL server uses two distinct programs: postmaster and postgres. The postmaster
process accepts all requests for database access. It does authentication and starts a postgres
process to handle the connection. The postgres process executes user queries and returns
results. Figure 20.4 illustrates this relationship.
POSTGRESQL sites normally have many postgres processes, but only one postmaster process.
One postgres process exists for every open database session.
Once administrators start the postmaster, the postmaster will start postgres backends as connection requests arrive. The postmaster can be started from either the command line or a script.
The operating system start-up scripts can even be modified to start the postmaster automatically.
A third way to start the postmaster is using pg_ctl. This utility allows easy starting and stopping
of the postmaster. See the pg_ctl manual page for more information.
To stop the postmaster, you can send the process a signal using kill, or use pg_ctl.

20.7. MONITORING

229
Connection Requests

postmaster

Queries

postgres

Results

postgres

postgres

Figure 20.4: Postmaster and postgres processes

20.7

Monitoring

Both postmaster and postgres produce useful information for administrators. They have many flags
to control the information they output. If desired, they can show user connection information, SQL
queries, and detailed performance statistics.
When the postmaster starts, its output should be sent to a file in the POSTGRESQL home
directory. That file can then be used to monitor database activity. See the postmaster and postgres
manual pages for a complete list of output options. To specify the flags to be passed to each
postgres process, use the postmaster -o flag.
Another way to monitor the database is by using ps. The ps operating system command displays
information about processes, including data about the postmaster and postgres processes. This
tool is helpful for analyzing POSTGRESQL activity, particularly for diagnosing problems. The ps
command can display information about the following aspects of a process:
• Current CPU usage
• Total CPU usage
• Start time
• Memory usage
• Disk operations (on some operating systems)
Each operating system uses different ps flags to output these values. A typical display is
USER
PID %CPU
…
postgres 18923 45.4

TIME STARTED
0:27.79 1:15PM

VSZ INBLK OUBLK COMMAND
2140

34

1 /usr/local/postgres/ …

CHAPTER 20. ADMINISTRATION

230

In this case, process 18923 is using 45.4% of the CPU, has used 27.79 seconds of CPU time, was
started at 1:15 P.M., has read 34 blocks, and has written 1 block.
To identify who is using each postgres process, most operating systems allow ps to display the
following connection information:
• User name
• User’s network address
• Database
• SQL command keyword (SELECT, INSERT, UPDATE, DELETE, CREATE, idle, …)
The ps command displays this information next to the name of each postgres process. A typical
display is
PID TT STAT
…
18923 ?? S
…

In this example, demouser, using process ID 18923, is connecting from the local machine to
database test and executing a SELECT. Administrators can use ps to analyze who is connected to
each database, the query command being run, and the system resources used.

20.8

Performance

Chapter 11 covered the performance of SQL queries. This chapter discusses more general performance considerations.
One of the most important administrative tasks is the scheduling of the vacuumdb -a command,
which vacuums all databases. It should be run when the databases are least busy. Section 11.4
describes the purpose of vacuuming. Vacuum analyze should also be performed periodically; it is
covered in Section 11.5. The vacuumdb command can perform analyzing as well. See the vacuumdb
manual page for more information.
Both postmaster and postgres have several flags that can improve performance. In POSTGRESQL
release 7.0 and earlier, the postgres -F flag prevents the database server from flushing all data
to disk at the end of each transaction. This improves performance, but if the operating system
abnormally shuts down, the database can be left in an inconsistent state. Later releases may not
use this flag.
The postmaster -B flag controls the amount of shared buffer memory allocated. The postgres -S
flag controls the amount of sort memory allocated. While these flags consume system resources,
they also improve performance by reducing disk access.

You can also improve database performance by moving databases to different disk drives. This
strategy spreads disk access among multiple drives. The initlocation utility allows new database
locations to be created on different drives; createdb can then use these locations for new databases.
POSTGRESQL stores tables and indexes in operating system files. Using operating system
symbolic links, you can move databases, tables, and indexes to different disk drives, which often
improves performance.

20.9

System Tables

A great deal of data is stored in POSTGRESQL system tables. The names of these tables begin with
pg_. The tables contain information about data types, functions, operators, databases, users, and
groups. Table 20.1 shows the most commonly used tables.
The pg_log table is a binary file rather than a real table. The pg_shadow table contains user
passwords and is not visible to ordinary users. The pg_user table (not listed in Table 20.1) is a view
of pg_shadow that does not display the password field. Several other system views are available
as well. Most system tables are joined using OIDs (see Section 7.1). Psqlâ&#x20AC;&#x2122;s \dS command lists all
system tables and views.

CHAPTER 20. ADMINISTRATION

232

20.10

Internationalization

POSTGRESQL supports several features important for international use. Multibyte encoding allows
non-ASCII character sets to be accurately stored in the database. It can be specified during
POSTGRESQL initialization, at database creation, or inside psql. POSTGRESQL can also be installed
to support international character sets, called locales.
POSTGRESQL can read and display dates in a variety of formats. The default date format can
be specified as a postgres flag, using SET DATESTYLE from inside psql, or using the PGDATESTYLE
environment variable.

20.11

Upgrading

The process of upgrading from previous POSTGRESQL releases is covered in the documentation
distributed with each version. Sometimes the pg_upgrade utility can be used. In other cases, a
pg_dumpall and reload are required.

20.12

Summary

This chapter merely summarizes the basic administrative tasks. Each utility has many other
options not covered here.
Administration can prove quite challenging. It takes skill and experience. This chapter has
supplied enough information that you should be able to start exploring topics of interest. The
manual pages and Administratorâ&#x20AC;&#x2122;s Guide mentioned in Appendix A.2 contain even more valuable
information.

Appendix A

Additional Resources
A.1

Mailing List Support

There are a variety of mailing lists available for discussing POSTGRESQL topics, getting help, and reporting bugs. This information can be found at http://www.postgresql.org/lists/mailing-list.html .

A.2 Supplied Documentation
POSTGRESQL comes with a variety of documentation. There is a general user guide, and specific
guides for administrators and programmers. There is also a tutorial. These can be all found at:
http://www.postgresql.org/docs/index.html .

A.4 Modifying the Source Code
POSTGRESQL allows users access to all of its source code. The Web page is at
http://www.postgresql.org/docs/index.html

It has a Developers section with many developer resources.
233

APPENDIX A. ADDITIONAL RESOURCES

234

A.5

Frequently Asked Questions (FAQs)

Frequently Asked Questions (FAQ) for POSTGRESQL
Last updated: Wed Jul 26 13:31:44 EDT 2000
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
The most recent version of this document can be viewed at
http://www.PostgreSQL.org/docs/faq-english.html .
Platform-specific questions are answered at http://www.PostgreSQL.org/docs/ .

General Questions
1.1) What is POSTGRESQL?
1.2) What’s the copyright on POSTGRESQL?
1.3) What Unix platforms does POSTGRESQL run on?
1.4) What non-Unix ports are available?
1.5) Where can I get POSTGRESQL?
1.6) Where can I get support?
1.7) What is the latest release?
1.8) What documentation is available?
1.9) How do I find out about known bugs or missing features?
1.10) How can I learn SQL?
1.11) Is POSTGRESQL Y2K compliant?
1.12) How do I join the development team?
1.13) How do I submit a bug report?
1.14) How does POSTGRESQL compare to other DBMS’s?

User Client Questions
2.1) Are there ODBC drivers for POSTGRESQL?
2.2) What tools are available for hooking POSTGRESQL to Web pages?
2.3) Does POSTGRESQL have a graphical user interface? A report generator? An embedded query
language interface?
2.4) What languages are available to communicate with POSTGRESQL?

Administrative Questions
3.1) Why does initdb fail?
3.2) How do I install POSTGRESQL somewhere other than /usr/local/pgsql?
3.3) When I start the postmaster, I get a Bad System Call or core dumped message. Why?
3.4) When I try to start the postmaster, I get IpcMemoryCreate errors. Why?
3.5) When I try to start the postmaster, I get IpcSemaphoreCreate errors. Why?

A.5. FREQUENTLY ASKED QUESTIONS (FAQS)

235

3.6) How do I prevent other hosts from accessing my POSTGRESQL database?
3.7) Why can’t I connect to my database from another machine?
3.8) Why can’t I access the database as the root user?
3.9) All my servers crash under concurrent table access. Why?
3.10) How do I tune the database engine for better performance?
3.11) What debugging features are available?
3.12) I get “Sorry, too many clients” when trying to connect. Why?
3.13) What are the pg_sorttempNNN.NN files in my database directory?

Operational Questions
4.1) Why is the system confused about commas, decimal points, and date formats?
4.2) What is the exact difference between binary cursors and normal cursors?
4.3) How do I SELECT only the first few rows of a query?
4.4) How do I get a list of tables or other things I can see in psql?
4.5) How do you remove a column from a table?
4.6) What is the maximum size for a row, table, database?
4.7) How much database disk space is required to store data from a typical text file?
4.8) How do I find out what indices or operations are defined in the database?
4.9) My queries are slow or don’t make use of the indexes. Why?
4.10) How do I see how the query optimizer is evaluating my query?
4.11) What is an R-tree index?
4.12) What is Genetic Query Optimization?
4.13) How do I do regular expression searches and case-insensitive regular expression searches?
4.14) In a query, how do I detect if a field is NULL?
4.15) What is the difference between the various character types?
4.16.1) How do I create a serial/auto-incrementing field?
4.16.2) How do I get the value of a SERIAL insert?
4.16.3) Don’t currval() and nextval() lead to a race condition with other users?
4.17) What is an OID? What is a TID?
4.18) What is the meaning of some of the terms used in POSTGRESQL?
4.19) Why do I get the error "FATAL: palloc failure: memory exhausted?"
4.20) How do I tell what POSTGRESQL version I am running?
4.21) My large-object operations get invalid large obj descriptor. Why?
4.22) How do I create a column that will default to the current time?
4.23) Why are my subqueries using IN so slow?
4.24) How do I do an outer join?

236

APPENDIX A. ADDITIONAL RESOURCES

Extending POSTGRESQL
5.1) I wrote a user-defined function. When I run it in psql, why does it dump core?
5.2) What does the message “NOTICE:PortalHeapMemoryFree: 0x402251d0 not in alloc set!”
mean?
5.3) How can I contribute some nifty new types and functions to POSTGRESQL?
5.4) How do I write a C function to return a tuple?
5.5) I have changed a source file. Why does the recompile not see the change?

General Questions
1.1) What is POSTGRESQL?
POSTGRESQL is an enhancement of the Postgres management system, a next-generation DBMS
research prototype. While POSTGRESQL retains the powerful data model and rich data types of
Postgres, it replaces the POSTQUEL query language with an extended subset of SQL. POSTGRESQL
is free and the complete source is available.
POSTGRESQL development is performed by a team of Internet developers who all subscribe
to the POSTGRESQL development mailing list. The current coordinator is Marc G. Fournier
(scrappy@PostgreSQL.org). (See below on how to join). This team is now responsible for all
development of POSTGRESQL.
The authors of POSTGRESQL 1.01 were Andrew Yu and Jolly Chen. Many others have contributed to the porting, testing, debugging, and enhancement of the code. The original Postgres
code, from which POSTGRESQL is derived, was the effort of many graduate students, undergraduate students, and staff programmers working under the direction of Professor Michael Stonebraker
at the University of California, Berkeley.
The original name of the software at Berkeley was Postgres. When SQL functionality was
added in 1995, its name was changed to Postgres95. The name was changed at the end of 1996 to
POSTGRESQL.
It is pronounced Post-Gres-Q-L.

1.2) What’s the copyright on POSTGRESQL?
POSTGRESQL is subject to the following COPYRIGHT:
POSTGRESQL Data Base Management System
Portions copyright (c) 1996-2000, PostgreSQL, Inc Portions Copyright (c) 1994-6 Regents of
the University of California
Permission to use, copy, modify, and distribute this software and its documentation for any
purpose, without fee, and without a written agreement is hereby granted, provided that the above
copyright notice and this paragraph and the following two paragraphs appear in all copies.

A.5. FREQUENTLY ASKED QUESTIONS (FAQS)

237

IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY
FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS
DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN ADVISED OF
THE POSSIBILITY OF SUCH DAMAGE.
THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE PROVIDED HEREUNDER
IS ON AN "AS IS" BASIS, AND THE UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS
TO PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.

1.4) What non-Unix ports are available?
It is possible to compile the libpq C library, psql, and other interfaces and binaries to run on MS
Windows platforms. In this case, the client is running on MS Windows, and communicates via
TCP/IP to a server running on one of our supported Unix platforms.
A file win31.mak is included in the distribution for making a Win32 libpq library and psql.
The database server is now working on Windows NT using the Cygnus Unix/NT porting library.
See pgsql/doc/FAQ_NT in the distribution.

1.5) Where can I get POSTGRESQL?
The primary anonymous ftp site for POSTGRESQL is ftp://ftp.PostgreSQL.org/pub. For mirror
sites, see our main Web site.

1.6) Where can I get support?
There is no support for POSTGRESQL from the University of California, Berkeley. It is maintained
through volunteer effort.
The main mailing list is: pgsql-general@PostgreSQL.org. It is available for discussion of
matters pertaining to POSTGRESQL. To subscribe, send mail with the following lines in the body
(not the subject line):
subscribe
end

to pgsql-general-request@PostgreSQL.org.
There is also a digest list available. To subscribe to this list, send email to: pgsql-generaldigest-request@PostgreSQL.org with a body of:
subscribe
end

Digests are sent out to members of this list whenever the main list has received around 30k of
messages.
The bugs mailing list is available. To subscribe to this list, send email to pgsql-bugsrequest@PostgreSQL.org with a body of:
subscribe
end

There is also a developers discussion mailing list available. To subscribe to this list, send email to
pgsql-hackers-request@PostgreSQL.org with a body of:

A.5. FREQUENTLY ASKED QUESTIONS (FAQS)

239

subscribe
end

Additional mailing lists and information about POSTGRESQL can be found via the POSTGRESQL
WWW home page at: http://www.PostgreSQL.org .
There is also an IRC channel on EFNet, channel #PostgreSQL. I use the Unix command irc
-c ’#PostgreSQL’ "$USER" irc.phoenix.net. Commercial support for POSTGRESQL is available at
http://www.pgsql.com/ .

1.7) What is the latest release?
The latest release of POSTGRESQL is version 7.0.2. We plan to have major releases every four
months.

1.8) What documentation is available?
Several manuals, manual pages, and some small test examples are included in the distribution. See
the /doc directory. You can also browse the manual online at http://www.postgresql.org/docs/postgres .
There is a POSTGRESQL book available at http://www.postgresql.org/docs/awbook.html .
psql has some nice backslash commands to show information about types, operators, functions,
aggregates, etc.
Our Web site contains even more documentation.

1.9) How do I find out about known bugs or missing features?
POSTGRESQL supports an extended subset of
features, and future plans.

SQL-92.

See our TODO list for known bugs, missing

1.10) How can I learn SQL?
The POSTGRESQL book at:
http://www.postgresql.org/docs/awbook.html

teaches SQL. There is a nice tutorial at
http://w3.one.net/˜jhoffman/sqltut.htm

and at
http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM .

Another one is Teach Yourself SQL in 21 Days, Second Edition at:
http://members.tripod.com/er4ebus/sql/index.htm

1.11) Is POSTGRESQL Y2K compliant?
Yes, we easily handle dates past the year 2000AD, and before 2000BC.

1.12) How do I join the development team?
First, download the latest source and read the POSTGRESQL Developers documentation on our
Web site, or in the distribution. Second, subscribe to the pgsql-hackers and pgsql-patches mailing
lists. Third, submit high-quality patches to pgsql-patches.
There are about a dozen people who have commit privileges to the POSTGRESQL CVS archive.
They each have submitted so many high-quality patches that it was impossible for the existing
committers to keep up, and we had confidence that patches they committed were of high quality.

1.13) How do I submit a bug report?
Fill out the bug-template file and send it to: pgsql-bugs@PostgreSQL.org
Also check out our ftp site, ftp://ftp.PostgreSQL.org/pub , to see if there is a more recent
POSTGRESQL version or patches.

1.14) How does POSTGRESQL compare to other DBMS’s?
There are several ways of measuring software: features, performance, reliability, support, and
price.
Features
POSTGRESQL has most features present in large commercial DBMS’s, like transactions, subselects,
triggers, views, foreign key referential integrity, and sophisticated locking. We have some features
they don’t have, like user-defined types, inheritance, rules, and multi-version concurrency control
to reduce lock contention. We don’t have outer joins, but are working on them.
Performance
POSTGRESQL runs in two modes. Normal fsync mode flushes every completed transaction to disk,
guaranteeing that if the OS crashes or loses power in the next few seconds, all your data is safely
stored on disk. In this mode, we are slower than most commercial databases, partly because few of
them do such conservative flushing to disk in their default modes. In no-fsync mode, we are usually
faster than commercial databases, though in this mode, an OS crash could cause data corruption.

A.5. FREQUENTLY ASKED QUESTIONS (FAQS)

241

We are working to provide an intermediate mode that suffers less performance overhead than full
fsync mode, and will allow data integrity within 30 seconds of an OS crash.
In comparison to MySQL or leaner database systems, we are slower on inserts/updates because
we have transaction overhead. Of course, MySQL doesn’t have any of the features mentioned
in the Features section above. We are built for flexibility and features, though we continue to
improve performance through profiling and source code analysis. There is an interesting Web page
comparing POSTGRESQL to MySQL at http://openacs.org/why-not-mysql.html .
We handle each user connection by creating a Unix process. Backend processes share data
buffers and locking information. With multiple CPU’s, multiple backends can easily run on different
CPU’s.
Reliability
We realize that a DBMS must be reliable, or it is worthless. We strive to release well-tested, stable
code that has a minimum of bugs. Each release has at least one month of beta testing, and our
release history shows that we can provide stable, solid releases that are ready for production use.
We believe we compare favorably to other database software in this area.
Support
Our mailing list provides a large group of developers and users to help resolve any problems
encountered. While we can not guarantee a fix, commercial DBMS’s don’t always supply a fix
either. Direct access to developers, the user community, manuals, and the source code often
make POSTGRESQL support superior to other DBMS’s. There is commercial per-incident support
available for those who need it. (See support FAQ item.)
Price
We are free for all use, both commercial and non-commercial. You can add our code to your product
with no limitations, except those outlined in our BSD-style license stated above.

User Client Questions
2.1) Are there ODBC drivers for POSTGRESQL?
There are two ODBC drivers available, PsqlODBC and OpenLink ODBC.
PsqlODBC is included in the distribution. More information about it can be gotten from:
ftp://ftp.PostgreSQL.org/pub/odbc/

APPENDIX A. ADDITIONAL RESOURCES

242

OpenLink ODBC can be gotten from http://www.openlinksw.com . It works with their standard ODBC
client software so you’ll have POSTGRESQL ODBC available on every client platform they support
(Win, Mac, Unix, VMS).
They will probably be selling this product to people who need commercial-quality support, but
a freeware version will always be available. Questions to postgres95@openlink.co.uk.
See also the ODBC chapter of the Programmer’s Guide.

2.2) What tools are available for hooking POSTGRESQL to Web pages?
A nice introduction to Database-backed Web pages can be seen at http://www.webtools.com .
There is also one at http://www.phone.net/home/mwm/hotlist/ .
For Web integration, PHP is an excellent interface. It is at http://www.php.net
For complex cases, many use the Perl interface and CGI.pm.
A WWW gateway based on WDB using Perl can be downloaded from
http://www.eol.ists.ca/dunlop/wdb-p95 .

2.3) Does POSTGRESQL have a graphical user interface? A report generator?
An embedded query language interface?
We have a nice graphical user interface called pgaccess, which is shipped as part of the distribution.
Pgaccess also has a report generator. The Web page is http://www.flex.ro/pgaccess .
We also include ecpg, which is an embedded SQL query language interface for C.

Administrative Questions
3.1) Why does initdb fail?
Try these:
• check that you don’t have any of the previous version’s binaries in your path
• check to see that you have the proper paths set
• check that the postgres user owns the proper files

3.2) How do I install POSTGRESQL somewhere other than /usr/local/pgsql?
The simplest way is to specify the –prefix option when running configure. If you forgot to do that,
you can edit Makefile.global and change POSTGRESDIR accordingly, or create a Makefile.custom and
define POSTGRESDIR there.

3.3) When I start the postmaster, I get a Bad System Call or core dumped
message. Why?
It could be a variety of problems, but first check to see that you have System V extensions installed
in your kernel. POSTGRESQL requires kernel support for shared memory and semaphores.

3.4) When I try to start the postmaster, I get IpcMemoryCreate errors. Why?
You either do not have shared memory configured properly in your kernel or you need to enlarge the
shared memory available in the kernel. The exact amount you need depends on your architecture
and how many buffers and backend processes you configure for the postmaster. For most systems,
with default numbers of buffers and processes, you need a minimum of ˜1MB.

3.5) When I try to start the postmaster, I get IpcSemaphoreCreate errors.
Why?
If the error message is IpcSemaphoreCreate: semget failed (No space left on device) then your kernel
is not configured with enough semaphores. Postgres needs one semaphore per potential backend
process. A temporary solution is to start the postmaster with a smaller limit on the number of
backend processes. Use -N with a parameter less than the default of 32. A more permanent
solution is to increase your kernel’s SEMMNS and SEMMNI parameters.
If the error message is something else, you might not have semaphore support configured in
your kernel at all.

244

APPENDIX A. ADDITIONAL RESOURCES

3.6) How do I prevent other hosts from accessing my POSTGRESQL database?
By default, POSTGRESQL only allows connections from the local machine using Unix domain
sockets. Other machines will not be able to connect unless you add the -i flag to the postmaster,
and enable host-based authentication by modifying the file $PGDATA/pg_hba.conf accordingly.
This will allow TCP/IP connections.

3.7) Why can’t I connect to my database from another machine?
The default configuration allows only Unix domain socket connections from the local machine. To
enable TCP/IP connections, make sure the postmaster has been started with the -i option, and add
an appropriate host entry to the file pgsql/data/pg_hba.conf.

3.8) Why can’t I access the database as the root user?
You should not create database users with user id 0 (root). They will be unable to access the
database. This is a security precaution because of the ability of users to dynamically link object
modules into the database engine.

3.9) All my servers crash under concurrent table access. Why?
This problem can be caused by a kernel that is not configured to support semaphores.

3.10) How do I tune the database engine for better performance?
Certainly, indices can speed up queries. The EXPLAIN command allows you to see how POSTGRESQL
is interpreting your query, and which indices are being used.
If you are doing a lot of INSERTs, consider doing them in a large batch using the COPY command.
This is much faster than individual INSERTs. Second, statements not in a BEGIN WORK/COMMIT
transaction block are considered to be in their own transaction. Consider performing several
statements in a single transaction block. This reduces the transaction overhead. Also consider
dropping and recreating indices when making large data changes.
There are several tuning options. You can disable fsync() by starting the postmaster with a -o -F
option. This will prevent fsync()’s from flushing to disk after every transaction.
You can also use the postmaster -B option to increase the number of shared memory buffers
used by the backend processes. If you make this parameter too high, the postmaster may not start
because you’ve exceeded your kernel’s limit on shared memory space. Each buffer is 8K and the
default is 64 buffers.
You can also use the backend -S option to increase the maximum amount of memory used by
the backend process for temporary sorts. The -S value is measured in kilobytes, and the default is
512 (ie, 512K).

A.5. FREQUENTLY ASKED QUESTIONS (FAQS)

245

You can also use the CLUSTER command to group data in tables to match an index. See the
page for more details.

CLUSTER manual

3.11) What debugging features are available?
POSTGRESQL has several features that report status information that can be valuable for debugging
purposes.
First, by running configure with the –enable-cassert option, many assert()’s monitor the progress
of the backend and halt the program when something unexpected occurs.
Both postmaster and postgres have several debug options available. First, whenever you start
the postmaster, make sure you send the standard output and error to a log file, like:
cd /usr/local/pgsql
./bin/postmaster > server.log 2>&1 &

This will put a server.log file in the top-level POSTGRESQL directory. This file contains useful
information about problems or errors encountered by the server. Postmaster has a -d option that
allows even more detailed information to be reported. The -d option takes a number that specifies
the debug level. Be warned that high debug level values generate large log files.
If the postmaster is not running, you can actually run the postgres backend from the command
line, and type your SQL statement directly. This is recommended only for debugging purposes.
Note that a newline terminates the query, not a semicolon. If you have compiled with debugging
symbols, you can use a debugger to see what is happening. Because the backend was not started
from the postmaster, it is not running in an identical environment and locking/backend interaction
problems may not be duplicated.
If the postmaster is running, start psql in one window, then find the PID of the postgres process
used by psql. Use a debugger to attach to the postgres PID. You can set breakpoints in the debugger
and issue queries from psql. If you are debugging postgres start-up, you can set PGOPTIONS="-W
n", then start psql. This will cause start-up to delay for n seconds so you can attach with the
debugger and trace through the start-up sequence.
The postgres program has -s, -A, and -t options that can be very useful for debugging and
performance measurements.
You can also compile with profiling to see what functions are taking execution time. The
backend profile files will be deposited in the pgsql/data/base/dbname directory. The client profile
file will be put in the client’s current directory.

3.12) I get “Sorry, too many clients” when trying to connect. Why?
You need to increase the postmaster’s limit on how many concurrent backend processes it can start.
In POSTGRESQL 6.5 and up, the default limit is 32 processes. You can increase it by restarting
the postmaster with a suitable -N value. With the default configuration you can set -N as large as

246

APPENDIX A. ADDITIONAL RESOURCES

1024. If you need more, increase MAXBACKENDS in include/config.h and rebuild. You can set the
default value of -N at configuration time, if you like, using configure’s –with-maxbackends switch.
Note that if you make -N larger than 32, you must also increase -B beyond its default of
64; -B must be at least twice -N, and probably should be more than that for best performance.
For large numbers of backend processes, you are also likely to find that you need to increase
various Unix kernel configuration parameters. Things to check include the maximum size of
shared memory blocks, SHMMAX; the maximum number of semaphores, SEMMNS and SEMMNI; the
maximum number of processes, NPROC; the maximum number of processes per user, MAXUPRC;
and the maximum number of open files, NFILE and NINODE. The reason that POSTGRESQL has a
limit on the number of allowed backend processes is so your system won’t run out of resources.
In POSTGRESQL versions prior to 6.5, the maximum number of backends was 64, and changing
it required a rebuild after altering the MaxBackendId constant in include/storage/sinvaladt.h.

3.13) What are the pg_sorttempNNN.NN files in my database directory?
They are temporary files generated by the query executor. For example, if a sort needs to be done
to satisfy an ORDER BY, and the sort requires more space than the backend’s -S parameter allows,
then temporary files are created to hold the extra data.
The temporary files should be deleted automatically, but might not if a backend crashes during
a sort. If you have no backends running at the time, it is safe to delete the pg_tempNNN.NN files.

Operational Questions
4.1) Why is the system confused about commas, decimal points, and date
formats?
Check your locale configuration. POSTGRESQL uses the locale setting of the user that ran the
postmaster process. There are postgres and psql SET commands to control the date format. Set
those accordingly for your operating environment.

4.2) What is the exact difference between binary cursors and normal cursors?
See the DECLARE manual page for a description.

4.3) How do I SELECT only the first few rows of a query?
See the FETCH manual page, or use SELECT…LIMIT….
The entire query may have to be evaluated, even if you only want the first few rows. Consider
a query that has an ORDER BY. If there is an index that matches the ORDER BY, POSTGRESQL may be
able to evaluate only the first few records requested, or the entire query may have to be evaluated
until the desired rows have been generated.

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4.4) How do I get a list of tables or other information I see in psql?
You can read the source code for psql in file pgsql/src/bin/psql/psql.c. It contains SQL commands that
generate the output for psql’s backslash commands. You can also start psql with the -E option so it
will print out the queries it uses to execute the commands you give.

4.5) How do you remove a column from a table?
We do not support ALTER TABLE DROP COLUMN, but do this:
SELECT ... -- select all columns but the one you want to remove
INTO TABLE new_table
FROM old_table;
DROP TABLE old_table;
ALTER TABLE new_table RENAME TO old_table;

4.6) What is the maximum size for a row, table, database?
These are the limits:
• Maximum size for a database? unlimited (60GB databases exist)
• Maximum size for a table? unlimited on all operating systems
• Maximum size for a row? 8k, configurable to 32k
• Maximum number of rows in a table? unlimited
• Maximum number of columns in a table? unlimited
• Maximum number of indexes on a table? unlimited
Of course, these are not actually unlimited, but limited to available disk space.
To change the maximum row size, edit include/config.h and change BLCKSZ. To use attributes
larger than 8K, you can also use the large object interface.
The row length limit will be removed in 7.1.

4.7) How much database disk space is required to store data from a typical
text file?
A POSTGRESQL database may need six-and-a-half times the disk space required to store the data
in a flat file.
Consider a file of 300,000 lines with two integers on each line. The flat file is 2.4MB. The size
of the POSTGRESQL database file containing this data can be estimated at 14MB:

Indexes do not require as much overhead, but do contain the data that is being indexed, so they
can be large also.

4.8) How do I find out what indices or operations are defined in the database?
psql has a variety of backslash commands to show such information. Use \ ? to see them.
Also try the file pgsql/src/tutorial/syscat.source. It illustrates many of the SELECTs needed to get
information from the database system tables.

4.9) My queries are slow or don’t make use of the indexes. Why?
POSTGRESQL does not automatically maintain statistics. VACUUM must be run to update the
statistics. After statistics are updated, the optimizer knows how many rows in the table, and can
better decide if it should use indices. Note that the optimizer does not use indices in cases when
the table is small because a sequential scan would be faster.
For column-specific optimization statistics, use VACUUM ANALYZE. VACUUM ANALYZE is important for complex multijoin queries, so the optimizer can estimate the number of rows returned
from each table, and choose the proper join order. The backend does not keep track of column
statistics on its own, so VACUUM ANALYZE must be run to collect them periodically.
Indexes are usually not used for ORDER BY operations: a sequential scan followed by an explicit
sort is faster than an indexscan of all tuples of a large table, because it takes fewer disk accesses.
When using wild-card operators such as LIKE or ˜, indices can only be used if the beginning of
the search is anchored to the start of the string. So, to use indices, LIKE searches should not begin
with %, and ˜(regular expression searches) should start with ˆ.

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249

4.10) How do I see how the query optimizer is evaluating my query?
See the EXPLAIN manual page.

4.11) What is an R-tree index?
An R-tree index is used for indexing spatial data. A hash index can’t handle range searches.
A B-tree index only handles range searches in a single dimension. R-tree’s can handle multidimensional data. For example, if an R-tree index can be built on an attribute of type point, the
system can more efficiently answer queries such as “select all points within a bounding rectangle.”
The canonical paper that describes the original R-tree design is:
Guttman, A. “R-trees: A Dynamic Index Structure for Spatial Searching.” Proc of the 1984
ACM SIGMOD Int’l Conf on Mgmt of Data, 45-57.
You can also find this paper in Stonebraker’s “Readings in Database Systems”
Built-in R-trees can handle polygons and boxes. In theory, R-trees can be extended to handle
higher number of dimensions. In practice, extending R-trees requires a bit of work and we don’t
currently have any documentation on how to do it.

4.12) What is Genetic Query Optimization?
The GEQO module speeds query optimization when joining many tables by means of a Genetic
Algorithm (GA). It allows the handling of large join queries through nonexhaustive search.

4.13) How do I do regular expression searches and case-insensitive regular
expression searches?
The ˜ operator does regular expression matching, and ˜* does case-insensitive regular expression
matching. There is no case-insensitive variant of the LIKE operator, but you can get the effect of
case-insensitive LIKE with this:
WHERE lower(textfield) LIKE lower(pattern)

4.14) In a query, how do I detect if a field is NULL?
You test the column with IS NULL and IS NOT NULL.

APPENDIX A. ADDITIONAL RESOURCES

250

4.15) What is the difference between the various character types?
Type
“CHAR”
CHAR(#)

Internal Name
Notes
char
1 character
bpchar
blank padded to the specified fixed length
size specifies maximum length, no padding
VARCHAR(#) varchar
text
length limited only by maximum row length
TEXT
BYTEA
bytea
variable-length array of bytes
You will see the internal name when examining system catalogs and in some error messages.
The last four types above are VARLENA types (i.e., the first four bytes are the length, followed
by the data). CHAR(#) allocates the maximum number of bytes no matter how much data is stored
in the field. TEXT, VARCHAR(#), and BYTEA all have variable length on the disk, and because of this,
there is a small performance penalty for using them. Specifically, the penalty is for access to all
columns after the first column of this type.

4.16.1) How do I create a serial/auto-incrementing field?
POSTGRESQL supports a
For example, this:

See the CREATE_SEQUENCE manual page for more information about sequences. You can also use
each row’s OID field as a unique value. However, if you need to dump and reload the database, you
need to use pg_dump’s -o option or COPY WITH OIDS option to preserve the OIDs.

4.16.2) How do I get the value of a SERIAL insert?
One approach is to to retrieve the next SERIAL value from the sequence object with the nextval()
function before inserting and then insert it explicitly. Using the example table in 4.16.1, that might
look like this:
$newSerialID = nextval(’person_id_seq’);
INSERT INTO person (id, name) VALUES ($newSerialID, ’Blaise Pascal’);

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251

You would then also have the new value stored in $newSerialID for use in other queries (e.g., as a
foreign key to the person table). Note that the name of the automatically created SEQUENCE object
will be named <table>_<serialcolumn>_seq, where table and serialcolumn are the names of your
table and your SERIAL column, respectively.
Alternatively, you could retrieve the assigned SERIAL value with the currval() function after it
was inserted by default, e.g.,
INSERT INTO person (name) VALUES (’Blaise Pascal’);
$newID = currval(’person_id_seq’);

Finally, you could use the OID returned from the INSERT statement to look up the default value,
though this is probably the least portable approach. In Perl, using DBI with Edmund Mergl’s
DBD::Pg module, the OID value is made available via $sth->{pg_oid_status} after $sth->execute().

4.16.3) Don’t currval() and nextval() lead to a race condition with other users?
No. This is handled by the backends.

4.17) What is an OID? What is a TID?
OIDs are POSTGRESQL’S answer to unique row ids. Every row that is created in POSTGRESQL gets
a unique OID. All OIDs generated during initdb are less than 16384 (from backend/access/transam.h).
All user-created OIDs are equal to or greater than this. By default, all these OIDs are unique not
only within a table or database, but unique within the entire POSTGRESQL installation.
POSTGRESQL uses OIDs in its internal system tables to link rows between tables. These OIDs
can be used to identify specific user rows and used in joins. It is recommended you use column
type OID to store OID values. You can create an index on the OID field for faster access.
OIDs are assigned to all new rows from a central area that is used by all databases. If you want
to change the OID to something else, or if you want to make a copy of the table, with the original
OID’s, there is no reason you can’t do it:
CREATE TABLE new_table(old_oid oid, mycol int);
SELECT old_oid, mycol INTO new FROM old;
COPY new TO ’/tmp/pgtable’;
DELETE FROM new;
COPY new WITH OIDS FROM ’/tmp/pgtable’;

TIDs are used to identify specific physical rows with block and offset values. TIDs change after
rows are modified or reloaded. They are used by index entries to point to physical rows.

APPENDIX A. ADDITIONAL RESOURCES

252

4.18) What is the meaning of some of the terms used in POSTGRESQL?
Some of the source code and older documentation use terms that have more common usage. Here
are some:
• table, relation, class
• row, record, tuple
• column, field, attribute
• retrieve, select
• replace, update
• append, insert
•

OID,

serial value

• portal, cursor
• range variable, table name, table alias

4.19) Why do I get the error “FATAL: palloc failure: memory exhausted?”
It is possible you have run out of virtual memory on your system, or your kernel has a low limit
for certain resources. Try this before starting the postmaster:
ulimit -d 65536
limit datasize 64m

Depending on your shell, only one of these may succeed, but it will set your process data segment
limit much higher and perhaps allow the query to complete. This command applies to the current
process, and all subprocesses created after the command is run. If you are having a problem with
the SQL client because the backend is returning too much data, try it before starting the client.

4.20) How do I tell what POSTGRESQL version I am running?
From psql, type select version();

4.21) My large-object operations get invalid large obj descriptor. Why?
You need to put BEGIN WORK and COMMIT around any use of a large object handle, that is, surrounding
lo_open ... lo_close. CurrentLY POSTGRESQL enforces the rule by closing large object handles
at transaction commit. So the first attempt to do anything with the handle will draw invalid large
obj descriptor. So code that used to work (at least most of the time) will now generate that error
message if you fail to use a transaction.
If you are using a client interface like ODBC you may need to set auto-commit off.

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253

4.22) How do I create a column that will default to the current time?
Use now():
CREATE TABLE test (x int, modtime timestamp DEFAULT now() );

4.23) Why are my subqueries using IN so slow?
Currently, we join subqueries to outer queries by sequentially scanning the result of the subquery
for each row of the outer query. A workaround is to replace IN with EXISTS:
SELECT * FROM tab WHERE col1 IN (SELECT col2 FROM TAB2)

4.24) How do I do an outer join?
POSTGRESQL does not support outer joins in the current release. They can be simulated using
UNION and NOT IN. For example, when joining tab1 and tab2, the following query does an outer join
of the two tables:
SELECT tab1.col1, tab2.col2
FROM tab1, tab2
WHERE tab1.col1 = tab2.col1
UNION ALL
SELECT tab1.col1, NULL
FROM tab1
WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
ORDER BY tab1.col1

Extending POSTGRESQL
5.1) I wrote a user-defined function. When I run it in psql, why does it dump
core?
The problem could be a number of things. Try testing your user-defined function in a stand-alone
test program first.

254

APPENDIX A. ADDITIONAL RESOURCES

5.2) What does the message “NOTICE:PortalHeapMemoryFree: 0x402251d0
not in alloc set!” mean?
You are pfree’ing something that was not palloc’ed. Beware of mixing malloc/free and palloc/pfree.

5.3) How can I contribute some nifty new types and functions to POSTGRESQL?
Send your extensions to the pgsql-hackers mailing list, and they will eventually end up in the
contrib/ subdirectory.

5.4) How do I write a C function to return a tuple?
This requires wizardry so extreme that the authors have never tried it, though in principle it can
be done.

5.5) I have changed a source file. Why does the recompile not see the change?
The Makefiles do not have the proper dependencies for include files. You have to do a make clean
and then another make.

Appendix B

Installation
Getting POSTGRESQL
The POSTGRESQL software is distributed in several formats:
• Tar-gzipped file with a file extension of .tar.gz
• Prepackaged file with a file extension of .rpm
• Another prepackaged format
• CD-ROM
Because so many formats exist, this appendix will cover only the general steps needed to install POSTGRESQL. Each distribution comes with an INSTALL or README file with more specific
instructions.

Creating the POSTGRESQL User
It is best to create a separate user to own the POSTGRESQL files and processes that will be installed.
The user name is typically postgres.

Configuration
Many distributions use a configure command that allows users to choose various options before
compiling and installing the software.

Compilation
POSTGRESQL is usually distributed in source code format. As a consequence, C source code must
be compiled into a format that is understood by the CPU. This process is usually handled by a
255

256

APPENDIX B. INSTALLATION

compiler, often called cc or gcc. Several distribution formats automatically perform these steps for
the user.

Installation
The installation process involves copying all compiled programs into a directory that will serve as
the home of all POSTGRESQL activity. It will also contain all POSTGRESQL programs, databases,
and log files. The directory is typically called /usr/local/pgsql.

Initialization
Initialization creates a database called template1 in the POSTGRESQL home directory. This database
is used to create all other databases. Initdb performs this initialization step.

Starting the Server
Once template1 is created, the database server can be started. This step typically involves running
the program called postmaster.

Creating a Database
Once the database server is running, you can create databases by running createdb from the
operating system prompt. Chapter 20 covers POSTGRESQL administration in detail.

Appendix C

PostgreSQL Nonstandard Features
by Chapter
This appendix outlines the nonstandard features covered in this book.
Chapter 1 None.
Chapter 2 Psql is a unique feature of POSTGRESQL.
Chapter 3 None.
Chapter 4 Use of regular expressions, SET, SHOW, and RESET are features unique to POSTGRESQL.
Chapter 5 None.
Chapter 6 None.
Chapter 7 OIDs, sequences, and SERIAL are unique features of POSTGRESQL.
Chapter 8 FROM in UPDATE is a unique feature of POSTGRESQL. Some databases support the
creation of tables by SELECT.
Chapter 9 Most databases support only a few of the many data types, functions, and operators
included in POSTGRESQL. Arrays are a unique features of POSTGRESQL. Large objects are
implemented differently by other database systems.
Chapter 10 None.
Chapter 11 CLUSTER, VACUUM, and EXPLAIN are features unique to POSTGRESQL.
Chapter 12 LIMIT is implemented by a few other database systems.
Chapter 13 Inheritance, RULES, LISTEN, and NOTIFY are features unique to POSTGRESQL.
257

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APPENDIX C. POSTGRESQL NONSTANDARD FEATURES BY CHAPTER

Chapter 14 None.
Chapter 15 COPY is a unique feature of POSTGRESQL.
Chapter 16 Psql and pgaccess are unique features of POSTGRESQL.
Chapter 17 All of the programming interfaces except ECPG and Java are implemented differently
by other database systems.
Chapter 18 Server-side functions and triggers are implemented differently by other database
systems.
Chapter 19 Using C to enhance the database is a unique POSTGRESQL feature.
Chapter 20 The administrative utilities are unique to POSTGRESQL.

Appendix D

Reference Manual
The following is a copy of the reference manual pages (man pages) as they appeared in POSTGRESQL
7.0.2. The most current version is available at:
• http://www.postgresql.org/docs/user/sql-commands.htm
• http://www.postgresql.org/docs/user/applications.htm
These are part of the POSTGRESQL User’s Guide.

D.1 ABORT
Name
ABORT — Aborts the current transaction
ABORT [ WORK | TRANSACTION ]
Inputs
None.
Outputs
ROLLBACK Message returned if successful.
NOTICE: ROLLBACK: no transaction in progress If there is not any transaction currently
in progress.

259

APPENDIX D. REFERENCE MANUAL

260

Description
ABORT rolls back the current transaction and causes all the updates made by the transaction to
be discarded. This command is identical in behavior to the SQL92 command ROLLBACK, and is
present only for historical reasons.
Notes
Use COMMIT to successfully terminate a transaction.

Usage
To abort all changes:
ABORT WORK;

Compatibility
SQL92
This command is a Postgres extension present for historical reasons. ROLLBACK is the SQL92
equivalent command.

D.2

ALTER GROUP

Name
ALTER GROUP â&#x20AC;&#x201D; Add users to a group, remove users from a group
ALTER GROUP name ADD USER username [, ... ]
ALTER GROUP name DROP USER username [, ... ]
Inputs
name The name of the group to modify.
username Users which are to be added or removed from the group. The user names must exist.
Outputs
ALTER GROUP Message returned if the alteration was successful.

D.3. ALTER TABLE

261

Description
ALTER GROUP is used to add or remove users from a group. Only database superusers can use
this command. Adding a user to a group does not create the user. Similarly, removing a user from
a group does not drop the user itself.
Use CREATE GROUP to create a new group and DROP GROUP to remove a group.

Usage
Add users to a group:
ALTER GROUP staff ADD USER karl, john

Remove a user from a group:
ALTER GROUP workers DROP USER beth

Compatibility
SQL92
There is no ALTER GROUP statement in SQL92. The concept of roles is similar.

newtable New name for the table.
table constraint definition New table constraint for the table
Outputs
ALTER Message returned from column or table renaming.
ERROR Message returned if table or column is not available.

Description
ALTER TABLE changes the definition of an existing table. The ADD COLUMN form adds a
new column to the table using the same syntax as CREATE TABLE. The ALTER COLUMN form
allows you to set or remove the default for the column. Note that defaults only apply to newly
inserted rows. The RENAME clause causes the name of a table or column to change without
changing any of the data contained in the affected table. Thus, the table or column will remain of
the same type and size after this command is executed. The ADD table constraint definition clause
adds a new constraint to the table using the same syntax as CREATE TABLE.
You must own the table in order to change its schema.
Notes
The keyword COLUMN is noise and can be omitted.
An asterisk (*) following a name of a table indicates that the statement should be run over that
table and all tables below it in the inheritance hierarchy; by default, the attribute will not be added
to or renamed in any of the subclasses. This should always be done when adding or modifying an
attribute in a superclass. If it is not, queries on the inheritance hierarchy such as SELECT NewColumn
FROM SuperClass * will not work because the subclasses will be missing an attribute found in the
superclass.
In the current implementation, default and constraint clauses for the new column will be
ignored. You can use the SET DEFAULT form of ALTER TABLE to set the default later. (You
will also have to update the already existing rows to the new default value, using UPDATE.)
In the current implementation, only FOREIGN KEY constraints can be added to a table. To
create or remove a unique constraint, create a unique index (see CREATE INDEX ). To add check
constraints you need to recreate and reload the table, using other parameters to the CREATE
TABLE command.
You must own the class in order to change its schema. Renaming any part of the schema
of a system catalog is not permitted. The PostgreSQL Userâ&#x20AC;&#x2122;s Guide has further information on
inheritance.
Refer to CREATE TABLE for a further description of valid arguments.

Compatibility
SQL92
The ADD COLUMN form is compliant with the exception that it does not support defaults and
constraints, as explained above. The ALTER COLUMN form is in full compliance.
SQL92 specifies some additional capabilities for ALTER TABLE statement which are not yet
directly supported by Postgres:
ALTER TABLE table DROP CONSTRAINT constraint { RESTRICT | CASCADE }
Removes a table constraint (such as a check constraint, unique constraint, or foreign key
constraint). To remove a unique constraint, drop a unique index. To remove other kinds of
constraints you need to recreate and reload the table, using other parameters to the CREATE
TABLE command.
For example, to drop any constraints on a table distributors:
CREATE TABLE temp AS
SELECT * FROM distributors;
DROP TABLE distributors;
CREATE TABLE distributors AS
SELECT * FROM temp;
DROP TABLE temp;

The clauses to rename columns and tables are Postgres extensions from SQL92.

D.4

ALTER USER

Name
ALTER USER — Modifies user account information
ALTER USER username
[ WITH PASSWORD ’password ’ ]
[ CREATEDB | NOCREATEDB ]
[ CREATEUSER | NOCREATEUSER ]
[ VALID UNTIL ’abstime ’ ]
Inputs
username The name of the user whose details are to be altered.
password The new password to be used for this account.
CREATEDB, NOCREATEDB These clauses define a user’s ability to create databases. If
CREATEDB is specified, the user being defined will be allowed to create his own databases.
Using NOCREATEDB will deny a user the ability to create databases.
CREATEUSER, NOCREATEUSER These clauses determine whether a user will be permitted
to create new users himself. This option will also make the user a superuser who can override
all access restrictions.
abstime The date (and, optionally, the time) at which this user’s password is to expire.

D.5. BEGIN

265

Outputs
ALTER USER Message returned if the alteration was successful.
ERROR: ALTER USER: user "username" does not exist Error message returned if the specified user is not known to the database.

Description
ALTER USER is used to change the attributes of a user’s Postgres account. Only a database
superuser can change privileges and password expiration with this command. Ordinary users can
only change their own password.
Use CREATE USER to create a new user and DROP USER to remove a user.

Change a user’s valid until date, specifying that his authorization should expire at midday on 4th
May 1998 using the time zone which is one hour ahead of UTC:
ALTER USER chris VALID UNTIL ’May 4 12:00:00 1998 +1’;

Give a user the ability to create other users and new databases:
ALTER USER miriam CREATEUSER CREATEDB;

Compatibility
SQL92
There is no ALTER USER statement in SQL92. The standard leaves the definition of users to
the implementation.

WORK, TRANSACTION Optional keywords. They have no effect.
Outputs
BEGIN This signifies that a new transaction has been started.
NOTICE: BEGIN: already a transaction in progress This indicates that a transaction was already in progress. The current transaction is not affected.

Description
By default, Postgres executes transactions in unchained mode (also known as autocommit in other
database systems). In other words, each user statement is executed in its own transaction and a
commit is implicitly performed at the end of the statement (if execution was successful, otherwise a
rollback is done). BEGIN initiates a user transaction in chained mode, i.e., all user statements after
BEGIN command will be executed in a single transaction until an explicit COMMIT, ROLLBACK,
or execution abort. Statements in chained mode are executed much faster, because transaction
start/commit requires significant CPU and disk activity. Execution of multiple statements inside a
transaction is also required for consistency when changing several related tables.
The default transaction isolation level in Postgres is READ COMMITTED, where queries inside
the transaction see only changes committed before query execution. So, you have to use SET
TRANSACTION ISOLATION LEVEL SERIALIZABLE just after BEGIN if you need more
rigorous transaction isolation. In SERIALIZABLE mode queries will see only changes committed
before the entire transaction began (actually, before execution of the first DML statement in a
serializable transaction).
If the transaction is committed, Postgres will ensure either that all updates are done or else
that none of them are done. Transactions have the standard ACID (atomic, consistent, isolatable,
and durable) property.
Notes
Refer to LOCK for further information about locking tables inside a transaction.
Use COMMIT or ROLLBACK to terminate a transaction.

Usage
To begin a user transaction:
BEGIN WORK;

D.6. CLOSE

267

Compatibility
SQL92
BEGIN is a Postgres language extension. There is no explicit BEGIN command in SQL92;
transaction initiation is always implicit and it terminates either with a COMMIT or ROLLBACK
statement. Note: Many relational database systems offer an autocommit feature as a convenience.
Incidentally, the BEGIN keyword is used for a different purpose in embedded SQL. You are
advised to be careful about the transaction semantics when porting database applications.
SQL92 also requires SERIALIZABLE to be the default transaction isolation level.

D.6 CLOSE
Name
CLOSE â&#x20AC;&#x201D; Close a cursor
CLOSE cursor
Inputs
cursor The name of an open cursor to close.
Outputs
CLOSE Message returned if the cursor is successfully closed.
NOTICE PerformPortalClose: portal "cursor" not found This warning is given if cursor is not
declared or has already been closed.

Description
CLOSE frees the resources associated with an open cursor. After the cursor is closed, no
subsequent operations are allowed on it. A cursor should be closed when it is no longer needed.
An implicit close is executed for every open cursor when a transaction is terminated by
COMMIT or ROLLBACK.
Notes
Postgres does not have an explicit OPEN cursor statement; a cursor is considered open when it
is declared. Use the DECLARE statement to declare a cursor.

APPENDIX D. REFERENCE MANUAL

268

Usage
Close the cursor liahona:
CLOSE liahona;

Compatibility
SQL92
CLOSE is fully compatible with SQL92.

D.7

CLUSTER

Name
CLUSTER â&#x20AC;&#x201D; Gives storage clustering advice to the server
CLUSTER indexname ON table
Inputs
indexname The name of an index.
table The name of a table.
Outputs
CLUSTER The clustering was done successfully.
ERROR: relation <tablerelation_number> inherits "table"
ERROR: Relation table does not exist!

Description
CLUSTER instructs Postgres to cluster the class specified by table approximately based on the
index specified by indexname. The index must already have been defined on classname.
When a class is clustered, it is physically reordered based on the index information. The
clustering is static. In other words, as the class is updated, the changes are not clustered. No
attempt is made to keep new instances or updated tuples clustered. If one wishes, one can
re-cluster manually by issuing the command again.

D.7. CLUSTER

269

Notes
The table is actually copied to a temporary table in index order, then renamed back to the original
name. For this reason, all grant permissions and other indexes are lost when clustering is
performed.
In cases where you are accessing single rows randomly within a table, the actual order of the
data in the heap table is unimportant. However, if you tend to access some data more than others,
and there is an index that groups them together, you will benefit from using CLUSTER.
Another place where CLUSTER is helpful is in cases where you use an index to pull out several
rows from a table. If you are requesting a range of indexed values from a table, or a single indexed
value that has multiple rows that match, CLUSTER will help because once the index identifies
the heap page for the first row that matches, all other rows that match are probably already on the
same heap page, saving disk accesses and speeding up the query.
There are two ways to cluster data. The first is with the CLUSTER command, which reorders
the original table with the ordering of the index you specify. This can be slow on large tables
because the rows are fetched from the heap in index order, and if the heap table is unordered, the
entries are on random pages, so there is one disk page retrieved for every row moved. Postgres
has a cache, but the majority of a big table will not fit in the cache.
Another way to cluster data is to use:
SELECT columnlist INTO TABLE newtable
FROM table ORDER BY columnlist

which uses the Postgres sorting code in the ORDER BY clause to match the index, and which is
much faster for unordered data. You then drop the old table, use ALTER TABLE/RENAME to
rename temp to the old name, and recreate any indexes. The only problem is that OIDs will not
be preserved. From then on, CLUSTER should be fast because most of the heap data has already
been ordered, and the existing index is used.

Usage
Cluster the employees relation on the basis of its salary attribute:
CLUSTER emp_ind ON emp;

D.9 COMMIT
Name
COMMIT — Commits the current transaction
COMMIT [ WORK | TRANSACTION ]
Inputs
WORK, TRANSACTION Optional keywords. They have no effect.
Outputs
COMMIT Message returned if the transaction is successfully committed.
NOTICE: COMMIT: no transaction in progress If there is no transaction in progress.

Description
COMMIT commits the current transaction. All changes made by the transaction become visible
to others and are guaranteed to be durable if a crash occurs.
Notes
The keywords WORK and TRANSACTION are noise and can be omitted.
Use ROLLBACK to abort a transaction.

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272

Usage
To make all changes permanent:
COMMIT WORK;

Compatibility
SQL92
SQL92 only specifies the two forms COMMIT and COMMIT WORK. Otherwise full compatibility.

D.10

COPY

Name
COPY — Copies data between files and tables
COPY [ BINARY ] table [ WITH OIDS ]
FROM { ’filename ’ | stdin } [ [USING] DELIMITERS ’delimiter ’ ] [ WITH NULL AS ’null
string ’ ]
COPY [ BINARY ] table [ WITH OIDS ]
TO { ’filename ’ | stdout } [ [USING] DELIMITERS ’delimiter ’ ] [ WITH NULL AS ’null string
’]
Inputs
BINARY Changes the behavior of field formatting, forcing all data to be stored or read in binary
format rather than as text.
table The name of an existing table.
WITH OIDS Copies the internal unique object id (OID) for each row.
filename The absolute Unix pathname of the input or output file.
stdin Specifies that input comes from a pipe or terminal.
stdout Specifies that output goes to a pipe or terminal.
delimiter A character that delimits the input or output fields.

D.10. COPY

273

null print A string to represent NULL values. The default is \N (backslash-N). You might prefer
an empty string, for example.
Note: On a copy in, any data item that matches this string will be stored as a NULL value,
so you should make sure that you use the same string as you used on copy out.
Outputs
COPY The copy completed successfully.
ERROR: reason The copy failed for the reason stated in the error message.

Description
COPY moves data between Postgres tables and standard file-system files. COPY instructs the
Postgres backend to directly read from or write to a file. The file must be directly visible to the
backend and the name must be specified from the viewpoint of the backend. If stdin or stdout are
specified, data flows through the client frontend to the backend.
Notes
The BINARY keyword will force all data to be stored/read as binary format rather than as text.
It is somewhat faster than the normal copy command, but is not generally portable, and the files
generated are somewhat larger, although this factor is highly dependent on the data itself.
By default, a text copy uses a tab (\t) character as a delimiter. The delimiter may also be
changed to any other single character with the keyword phrase USING DELIMITERS. Characters
in data fields which happen to match the delimiter character will be backslash quoted.
You must have select access on any table whose values are read by COPY, and either insert or
update access to a table into which values are being inserted by COPY. The backend also needs
appropriate Unix permissions for any file read or written by COPY.
The keyword phrase USING DELIMITERS specifies a single character to be used for all
delimiters between columns. If multiple characters are specified in the delimiter string, only the
first character is used. Tip: Do not confuse COPY with the psql instruction \copy.
COPY neither invokes rules nor acts on column defaults. It does invoke triggers, however.
COPY stops operation at the first error. This should not lead to problems in the event of
a COPY FROM, but the target relation will, of course, be partially modified in a COPY TO.
VACUUM should be used to clean up after a failed copy.
Because the Postgres backendâ&#x20AC;&#x2122;s current working directory is not usually the same as the userâ&#x20AC;&#x2122;s
working directory, the result of copying to a file "foo" (without additional path information) may
yield unexpected results for the naive user. In this case, foo will wind up in $PGDATA/foo. In
general, the full pathname as it would appear to the backend server machine should be used when
specifying files to be copied.

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Files used as arguments to COPY must reside on or be accessible to the database server
machine by being either on local disks or on a networked file system.
When a TCP/IP connection from one machine to another is used, and a target file is specified,
the target file will be written on the machine where the backend is running rather than the userâ&#x20AC;&#x2122;s
machine.

File Formats
Text Format
When COPY TO is used without the BINARY option, the file generated will have each row
(instance) on a single line, with each column (attribute) separated by the delimiter character.
Embedded delimiter characters will be preceded by a backslash character ( \ ). The attribute
values themselves are strings generated by the output function associated with each attribute
type. The output function for a type should not try to generate the backslash character; this will
be handled by COPY itself.
The actual format for each instance is <attr1> <separator> <attr2> <separator> ...<separator> <attrn> <newline>. The oid is placed on the beginning of the line if WITH OIDS is
specified.
If COPY is sending its output to standard output instead of a file, it will send a backslash ( \ )
and a period (.) followed immediately by a newline, on a separate line, when it is done. Similarly, if
COPY is reading from standard input, it will expect a backslash ( \ ) and a period (.) followed by a
newline, as the first three characters on a line to denote end-of-file. However, COPY will terminate
(followed by the backend itself) if a true EOF is encountered before this special end-of-file pattern
is found.
The backslash character has other special meanings. A literal backslash character is represented as two consecutive backslashes ( \ \ ). A literal tab character is represented as a backslash
and a tab. A literal newline character is represented as a backslash and a newline. When loading text data not generated by Postgres, you will need to convert backslash characters ( \ ) to
double-backslashes ( \ \ ) to ensure that they are loaded properly.
Binary Format
In the case of COPY BINARY, the first four bytes in the file will be the number of instances in the
file. If this number is zero, the COPY BINARY command will read until end-of-file is encountered.
Otherwise, it will stop reading when this number of instances has been read. Remaining data in
the file will be ignored.
The format for each instance in the file is as follows. Note that this format must be followed
exactly. Unsigned four-byte integer quantities are called uint32 in the table below.

D.10. COPY

275

Type
uint32
uint32
uint32
uint32
[uint32,...,uint32]

Contents of a binary copy file
Meaning
number of tuples
For every tuple
total length of tuple data
oid (if specified)
number of null attributes
attribute numbers of attributes, counting from 0

Alignment of Binary Data
On Sun-3s, 2-byte attributes are aligned on two-byte boundaries, and all larger attributes are
aligned on four-byte boundaries. Character attributes are aligned on single-byte boundaries. On
most other machines, all attributes larger than 1 byte are aligned on four-byte boundaries. Note
that variable length attributes are preceded by the attribute’s length; arrays are simply contiguous
streams of the array element type.

Usage
The following example copies a table to standard output, using a pipe (|) as the field delimiter:
COPY country TO stdout USING DELIMITERS ’|’;

To copy data from a Unix file into a table country:
COPY country FROM ’/usr1/proj/bray/sql/country_data’;

Here is a sample of data suitable for copying into a table from stdin (so it has the termination
sequence on the last line):
AF
AL
DZ
ZM
ZW
\.

AFGHANISTAN
ALBANIA
ALGERIA ...
ZAMBIA
ZIMBABWE

The following is the same data, output in binary format on a Linux/i586 machine. The data is
shown after filtering through the Unix utility od -c. The table has three fields; the first is char(2)
and the second is text. All the rows have a null value in the third field. Notice how the char(2)
field is padded with nulls to four bytes and the text field is preceded by its length:

Name
CREATE AGGREGATE — Defines a new aggregate function
CREATE AGGREGATE name (
BASETYPE = input_data_type
[ , SFUNC1 = sfunc1 , STYPE1 = state1_type ]
[ , SFUNC2 = sfunc2 , STYPE2 = state2_type ]
[ , FINALFUNC = ffunc ]
[ , INITCOND1 = initial_condition1 ]
[ , INITCOND2 = initial_condition2 ] )
Inputs
name The name of an aggregate function to create.
input_data_type The input data type on which this aggregate function operates.
sfunc1 A state transition function to be called for every non-NULL input data value. This must
be a function of two arguments, the first being of type state1_type and the second of type
input_data_type. The function must return a value of type state1_type. This function takes
the current state value 1 and the current input data item, and returns the next state value 1.
state1_type The data type for the first state value of the aggregate.

D.11. CREATE AGGREGATE

277

sfunc2 A state transition function to be called for every non-NULL input data value. This must
be a function of one argument of type state2_type, returning a value of the same type. This
function takes the current state value 2 and returns the next state value 2.
state2_type The data type for the second state value of the aggregate.
ffunc The final function called to compute the aggregate’s result after all input data has been
traversed. If both state values are used, the final function must take two arguments of types
state1_type and state2_type. If only one state value is used, the final function must take a
single argument of that state value’s type. The output data type of the aggregate is defined
as the return type of this function.
initial_condition1 The initial value for state value 1.
initial_condition2 The initial value for state value 2.
Outputs
CREATE Message returned if the command completes successfully.

Description
CREATE AGGREGATE allows a user or programmer to extend Postgres functionality by defining
new aggregate functions. Some aggregate functions for base types such as min(int4) and avg(float8)
are already provided in the base distribution. If one defines new types or needs an aggregate
function not already provided, then CREATE AGGREGATE can be used to provide the desired
features.
An aggregate function is identified by its name and input data type. Two aggregates can have
the same name if they operate on different input types. To avoid confusion, do not make an ordinary
function of the same name and input data type as an aggregate.
An aggregate function is made from between one and three ordinary functions: two state
transition functions, sfunc1 and sfunc2, and a final calculation function, ffunc. These are used as
follows:
sfunc1 ( internal-state1, next-data-item ) —> next-internal-state1
sfunc2 ( internal-state2 ) —> next-internal-state2
ffunc (internal-state1, internal-state2) —> aggregate-value
Postgres creates one or two temporary variables (of data types stype1 and/or stype2 ) to hold the
current internal states of the aggregate. At each input data item, the state transition function(s) are
invoked to calculate new values for the internal state values. After all the data has been processed,
the final function is invoked once to calculate the aggregate’s output value.

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278

ffunc must be specified if both transition functions are specified. If only one transition function
is used, then ffunc is optional. The default behavior when ffunc is not provided is to return the
ending value of the internal state value being used (and, therefore, the aggregate’s output type is
the same as that state value’s type).
An aggregate function may also provide one or two initial conditions, that is, initial values for
the internal state values being used. These are specified and stored in the database as fields of type
text, but they must be valid external representations of constants of the state value data types.
If sfunc1 is specified without an initcond1 value, then the system does not call sfunc1 at the first
input item; instead, the internal state value 1 is initialized with the first input value, and sfunc1 is
called beginning at the second input item. This is useful for aggregates like MIN and MAX. Note
that an aggregate using this feature will return NULL when called with no input values. There is
no comparable provision for state value 2; if sfunc2 is specified then an initcond2 is required.
Notes
Use DROP AGGREGATE to drop aggregate functions.
The parameters of CREATE AGGREGATE can be written in any order, not just the order
illustrated above.
It is possible to specify aggregate functions that have varying combinations of state and final
functions. For example, the count aggregate requires sfunc2 (an incrementing function) but not
sfunc1 or ffunc, whereas the sum aggregate requires sfunc1 (an addition function) but not sfunc2 or
ffunc, and the avg aggregate requires both state functions as well as a ffunc (a division function) to
produce its answer. In any case, at least one state function must be defined, and any sfunc2 must
have a corresponding initcond2.

Usage
Refer to the chapter on aggregate functions in the PostgreSQL Programmer’s Guide for complete
examples of usage.

Compatibility
SQL92
CREATE AGGREGATE is a Postgres language extension. There is no CREATE AGGREGATE
in SQL92.

D.12

CREATE CONSTRAINT TRIGGER

Name
CREATE CONSTRAINT TRIGGER — Create a trigger to support a constraint

D.13. CREATE DATABASE

279

CREATE CONSTRAINT TRIGGER name
AFTER events ON relation constraint attributes
FOR EACH ROW
EXECUTE PROCEDURE func ’(’ args ’)’
Inputs
name The name of the constraint trigger.
events The event categories for which this trigger should be fired.
relation Table name of the triggering relation.
constraint Actual constraint specification.
attributes Constraint attributes.
func (args ) Function to call as part of the trigger processing.
Outputs
CREATE CONSTRAINT Message returned if successful.

Description
CREATE CONSTRAINT TRIGGER is used from inside of CREATE/ALTER TABLE and by
pg_dump to create the special triggers for referential integrity.
It is not intended for general use.

D.13

CREATE DATABASE

Name
CREATE DATABASE — Creates a new database
CREATE DATABASE name [ WITH LOCATION = ’dbpath ’ ]
Inputs
name The name of a database to create.
dbpath An alternate location where to store the new database in the filesystem. See below for
caveats.

280

APPENDIX D. REFERENCE MANUAL

Outputs
CREATE DATABASE Message returned if the command completes successfully.
ERROR: user ’username ’ is not allowed to create/drop databases You must have the special CREATEDB privilege to create databases. See CREATE USER.
ERROR: createdb: database "name " already exists This occurs if a database with the name
specified already exists.
ERROR: Single quotes are not allowed in database names., ERROR: Single quotes are
not allowed in database paths. The database name and dbpath cannot contain single
quotes. This is required so that the shell commands that create the database directory can
execute safely.
ERROR: The path ’xxx’ is invalid. The expansion of the specified dbpath (see below) failed.
Check the path you entered or make sure that the environment variable you are referencing
does exist.
ERROR: createdb: May not be called in a transaction block. If you have an explicit transaction block in progress you cannot call CREATE DATABASE. You must finish the transaction
first.
ERROR: Unable to create database directory ’path ’., ERROR: Could not initialize database
directory. These are most likely related to insufficient permissions on the data directory, a
full disk, or other file system problems. The user under which the database server is running
must have access to the location.

Description
CREATE DATABASE creates a new Postgres database. The creator becomes the owner of the
new database.
An alternate location can be specified in order to, for example, store the database on a different
disk. The path must have been prepared with the initlocation command.
If the path contains a slash, the leading part is interpreted as an environment variable, which
must be known to the server process. This way the database administrator can exercise control
over at which locations databases can be created. (A customary choice is, e.g., ’PGDATA2’.) If
the server is compiled with ALLOW_ABSOLUTE_DBPATHS (not so by default), absolute path
names, as identified by a leading slash (e.g., ’/usr/local/pgsql/data’), are allowed as well.
Notes
CREATE DATABASE is a Postgres language extension.
Use DROP DATABASE to remove a database.

D.14. CREATE FUNCTION

281

The program createdb is a shell script wrapper around this command, provided for convenience.
There are security and data integrity issues involved with using alternate database locations
specified with absolute path names, and by default only an environment variable known to the
backend may be specified for an alternate location. See the Administrator’s Guide for more
information.

Usage
To create a new database:
olly=>

create database lusiadas;

To create a new database in an alternate area /private_db:
$ mkdir private_db
$ initlocation /private_db

Compatibility
SQL92
There is no CREATE DATABASE statement in SQL92. Databases are equivalent to catalogs
whose creation is implementation-defined.

D.14

CREATE FUNCTION

Name
CREATE FUNCTION — Defines a new function

282

APPENDIX D. REFERENCE MANUAL

CREATE FUNCTION name ( [ ftype [, ...] ] )
RETURNS rtype
AS definition
LANGUAGE ’langname ’ [ WITH ( attribute [, ...] ) ]
CREATE FUNCTION name ( [ ftype [, ...] ] )
RETURNS rtype
AS obj_file , link_symbol
LANGUAGE ’C’ [ WITH ( attribute [, ...] ) ]
Inputs
name The name of a function to create.
ftype The data type of function arguments. The input types may be base or complex types, or
opaque. Opaque indicates that the function accepts arguments of an invalid type such as char
*.
rtype The return data type. The output type may be specified as a base type, complex type, setof
type, or opaque. The setof modifier indicates that the function will return a set of items,
rather than a single item.
attribute An optional piece of information about the function, used for optimization. The only
attribute currently supported is iscachable. Iscachable indicates that the function always
returns the same result when given the same input values (i.e., it does not do database lookups
or otherwise use information not directly present in its parameter list). The optimizer uses
iscachable to know whether it is safe to pre-evaluate a call of the function.
definition A string defining the function; the meaning depends on the language. It may be an
internal function name, the path to an object file, an SQL query, or text in a procedural
language.
obj_file , link_symbol This form of the AS clause is used for dynamically linked, C language
functions when the function name in the C language source code is not the same as the name
of the SQL function. The string obj_file is the name of the file containing the dynamically
loadable object, and link_symbol, is the object’s link symbol which is the same as the name
of the function in the C language source code.
langname May be ’C’, ’sql’, ’internal’ or ’plname’, where ’plname’ is the name of a created
procedural language. See CREATE LANGUAGE for details.
Outputs
CREATE This is returned if the command completes successfully.

D.14. CREATE FUNCTION

283

Description
CREATE FUNCTION allows a Postgres user to register a function with a database. Subsequently,
this user is considered the owner of the function.
Notes
Refer to the chapter in the PostgreSQL Programmer’s Guide on the topic of extending Postgres via
functions for further information on writing external functions.
Use DROP FUNCTION to remove user-defined functions.
Postgres allows function “overloading”; that is, the same name can be used for several different
functions so long as they have distinct argument types. This facility must be used with caution for
internal and C-language functions, however.
The full SQL92 type syntax is allowed for input arguments and return value. However, some
details of the type specification (e.g., the precision field for numeric types) are the responsibility
of the underlying function implementation and are silently swallowed (e.g., not recognized or
enforced) by the CREATE FUNCTION command.
Two internal functions cannot have the same C name without causing errors at link time. To
get around that, give them different C names (for example, use the argument types as part of the
C names), then specify those names in the AS clause of CREATE FUNCTION. If the AS clause
is left empty, then CREATE FUNCTION assumes the C name of the function is the same as the
SQL name.
When overloading SQL functions with C-language functions, give each C-language instance
of the function a distinct name, and use the alternative form of the AS clause in the CREATE
FUNCTION syntax to ensure that overloaded SQL functions names are resolved to the correct
dynamically linked objects.
A C function cannot return a set of values.

This example creates a C function by calling a routine from a user-created shared library. This
particular routine calculates a check digit and returns TRUE if the check digit in the function
parameters is correct. It is intended for use in a CHECK constraint.

This example creates a function that does type conversion between the user-defined type complex,
and the internal type point. The function is implemented by a dynamically loaded object that was
compiled from C source. For Postgres to find a type conversion function automatically, the sql
function has to have the same name as the return type, and overloading is unavoidable. The
function name is overloaded by using the second form of the AS clause in the SQL definition:
CREATE FUNCTION point(complex) RETURNS point
AS ’/home/bernie/pgsql/lib/complex.so’, ’complex_to_point’
LANGUAGE ’c’;

SQL/PSM
Note: PSM stands for Persistent Stored Modules. It is a procedural language and it was originally
hoped that PSM would be ratified as an official standard by late 1996. As of mid-1998, this has not
yet happened, but it is hoped that PSM will eventually become a standard. SQL/PSM CREATE
FUNCTION has the following syntax:
CREATE FUNCTION name
([ [ IN | OUT | INOUT ] type [, ...] ] )
RETURNS rtype
LANGUAGE ’langname’
ESPECIFIC routine
SQL-statement

D.15

CREATE GROUP

Name
CREATE GROUP — Creates a new group
CREATE GROUP name
[ WITH [ SYSID gid ]
[ USER username [, ...] ] ]
Inputs
name The name of the group.
gid The SYSID clause can be used to choose the Postgres group id of the new group. It is not
necessary to do so, however.
If this is not specified, the highest assigned group id plus one, starting at 1, will be used as
default.
username A list of users to include in the group. The users must already exist.
Outputs
CREATE GROUP Message returned if the command completes successfully.

Description
CREATE GROUP will create a new group in the database installation. Refer to the administrator’s
guide for information about using groups for authentication. You must be a database superuser to
use this command.

APPENDIX D. REFERENCE MANUAL

286

Use ALTER GROUP to change a group’s membership, and DROP GROUP to remove a group.

Usage
Create an empty group:
CREATE GROUP staff

Create a group with members:
CREATE GROUP marketing WITH USER jonathan, david

Compatibility
SQL92
There is no CREATE GROUP statement in SQL92. Roles are similar in concept to groups.

D.16

CREATE INDEX

Name
CREATE INDEX — Constructs a secondary index
CREATE [ UNIQUE ] INDEX index_name ON table
[ USING acc_name ] ( column [ ops_name ] [, ...] )
CREATE [ UNIQUE ] INDEX index_name ON table
[ USING acc_name ] ( func_name ( column [, ... ]) [ ops_name ] )
Inputs
UNIQUE Causes the system to check for duplicate values in the table when the index is created
(if data already exist) and each time data is added. Attempts to insert or update data which
would result in duplicate entries will generate an error.
index_name The name of the index to be created.
table The name of the table to be indexed.
acc_name The name of the access method to be used for the index. The default access method
is BTREE. Postgres provides three access methods for indexes:
BTREE an implementation of Lehman-Yao high-concurrency btrees.
RTREE implements standard rtrees using Guttman’s quadratic split algorithm.

D.16. CREATE INDEX

287

HASH an implementation of Litwin’s linear hashing.
column The name of a column of the table.
ops_name An associated operator class. See below for details.
func_name A function, which returns a value that can be indexed.
Outputs
CREATE The message returned if the index is successfully created.
ERROR: Cannot create index: ’index_name’ already exists. This error occurs if it is impossible to create the index.

Description
CREATE INDEX constructs an index index_name on the specified table. Tip: Indexes are
primarily used to enhance database performance. But inappropriate use will result in slower
performance.
In the first syntax shown above, the key field(s) for the index are specified as column names.
Multiple fields can be specified if the index access method supports multi-column indexes.
In the second syntax shown above, an index is defined on the result of a user-specified function
func_name applied to one or more attributes of a single class. These functional indices can be used
to obtain fast access to data based on operators that would normally require some transformation
to apply them to the base data.
Postgres provides btree, rtree and hash access methods for indices. The btree access method
is an implementation of Lehman-Yao high-concurrency btrees. The rtree access method implements standard rtrees using Guttman’s quadratic split algorithm. The hash access method is an
implementation of Litwin’s linear hashing. We mention the algorithms used solely to indicate that
all of these access methods are fully dynamic and do not have to be optimized periodically (as is
the case with, for example, static hash access methods).
Use DROP INDEX to remove an index.
Notes
The Postgres query optimizer will consider using a btree index whenever an indexed attribute is
involved in a comparison using one of: <, <=, =, >=, >
The Postgres query optimizer will consider using an rtree index whenever an indexed attribute
is involved in a comparison using one of: << , & <, &> , >> , @, ˜=, &&
The Postgres query optimizer will consider using a hash index whenever an indexed attribute
is involved in a comparison using the = operator.

APPENDIX D. REFERENCE MANUAL

288

Currently, only the btree access method supports multi-column indexes. Up to 16 keys may
be specified by default (this limit can be altered when building Postgres).
An operator class can be specified for each column of an index. The operator class identifies
the operators to be used by the index for that column. For example, a btree index on four-byte
integers would use the int4_ops class; this operator class includes comparison functions for fourbyte integers. In practice the default operator class for the field’s data type is usually sufficient.
The main point of having operator classes is that for some data types, there could be more than
one meaningful ordering. For example, we might want to sort a complex-number data type either
by absolute value or by real part. We could do this by defining two operator classes for the data
type and then selecting the proper class when making an index. There are also some operator
classes with special purposes:
• The operator classes box_ops and bigbox_ops both support rtree indices on the box data
type. The difference between them is that bigbox_ops scales box coordinates down, to avoid
floating-point exceptions from doing multiplication, addition, and subtraction on very large
floating-point coordinates. If the field on which your rectangles lie is about 20,000 units
square or larger, you should use bigbox_ops.
• The int24_ops operator class is useful for constructing indices on int2 data, and doing
comparisons against int4 data in query qualifications. Similarly, int42_ops support indices on
int4 data that is to be compared against int2 data in queries.
The following query shows all defined operator classes:
SELECT am.amname AS acc_name,
opc.opcname AS ops_name,
opr.oprname AS ops_comp
FROM pg_am am, pg_amop amop, pg_opclass opc, pg_operator opr
WHERE amop.amopid = am.oid AND
amop.amopclaid = opc.oid AND
amop.amopopr = opr.oid
ORDER BY acc_name, ops_name, ops_comp

Usage
To create a btree index on the field title in the table films:
CREATE UNIQUE INDEX title_idx ON films (title);

Compatibility
SQL92
CREATE INDEX is a Postgres language extension.

D.17. CREATE LANGUAGE

289

There is no CREATE INDEX command in SQL92.

D.17

CREATE LANGUAGE

Name
CREATE LANGUAGE — Defines a new language for functions
CREATE [ TRUSTED ] PROCEDURAL LANGUAGE ’langname ’
HANDLER call_handler
LANCOMPILER ’comment ’
Inputs
TRUSTED TRUSTED specifies that the call handler for the language is safe; that is, it offers an
unprivileged user no functionality to bypass access restrictions. If this keyword is omitted
when registering the language, only users with the Postgres superuser privilege can use this
language to create new functions (like the C language).
langname The name of the new procedural language. The language name is case insensitive. A
procedural language cannot override one of the built-in languages of Postgres.
HANDLER call_handler call_handler is the name of a previously registered function that will
be called to execute the PL procedures.
comment The LANCOMPILER argument is the string that will be inserted in the LANCOMPILER attribute of the new pg_language entry. At present, Postgres does not use this
attribute in any way.
Outputs
CREATE This message is returned if the language is successfully created.
ERROR: PL handler function funcname () doesn’t exist This error is returned if the function funcname () is not found.

Description
Using CREATE LANGUAGE, a Postgres user can register a new language with Postgres. Subsequently, functions and trigger procedures can be defined in this new language. The user must
have the Postgres superuser privilege to register a new language.

APPENDIX D. REFERENCE MANUAL

290
Writing PL handlers

The call handler for a procedural language must be written in a compiler language such as C
and registered with Postgres as a function taking no arguments and returning the opaque type, a
placeholder for unspecified or undefined types. This prevents the call handler from being called
directly as a function from queries.
However, arguments must be supplied on the actual call when a PL function or trigger procedure
in the language offered by the handler is to be executed.
• When called from the trigger manager, the only argument is the object ID from the procedure’s pg_proc entry. All other information from the trigger manager is found in the global
CurrentTriggerData pointer.
• When called from the function manager, the arguments are the object ID of the procedure’s
pg_proc entry, the number of arguments given to the PL function, the arguments in a
FmgrValues structure, and a pointer to a boolean where the function tells the caller if the
return value is the SQL NULL value.
It’s up to the call handler to fetch the pg_proc entry and to analyze the argument and return types
of the called procedure. The AS clause from the CREATE FUNCTION of the procedure will
be found in the prosrc attribute of the pg_proc table entry. This may be the source text in the
procedural language itself (like for PL/Tcl), a pathname to a file, or anything else that tells the call
handler what to do in detail.
Notes
Use CREATE FUNCTION to create a function.
Use DROP LANGUAGE to drop procedural languages.
Refer to the table pg_language for further information:
Table "pg_language"
Attribute
| Type
| Modifier
---------------+---------+---------lanname
| name
|
lanispl
| boolean |
lanpltrusted | boolean |
lanplcallfoid | oid
|
lancompiler
| text
|
lanname | lanispl | lanpltrusted | lanplcallfoid | lancompiler
----------+---------+--------------+---------------+------------internal | f
| f
|
0 | n/a

D.17. CREATE LANGUAGE
C
sql

| f
| f

| f
| f

291
|
|

0 | /bin/cc
0 | postgres

Since the call handler for a procedural language must be registered with Postgres in the C language,
it inherits all the capabilities and restrictions of C functions.
At present, the definitions for a procedural language cannot be changed once they have been
created.

Only a few thousand lines of code have to be added instead of the dots to complete the PL call
handler. See CREATE FUNCTION for information on how to compile it into a loadable module.
The following commands then register the sample procedural language:
CREATE FUNCTION plsample_call_handler () RETURNS opaque
AS ’/usr/local/pgsql/lib/plsample.so’
LANGUAGE ’C’;
CREATE PROCEDURAL LANGUAGE ’plsample’
HANDLER plsample_call_handler
LANCOMPILER ’PL/Sample’;

Compatibility
SQL92
CREATE LANGUAGE is a Postgres extension. There is no CREATE LANGUAGE statement
in SQL92.

Inputs
name The operator to be defined. See below for allowable characters.
func_name The function used to implement this operator.
type1 The type of the left-hand argument of the operator, if any. This option would be omitted for
a left-unary operator.
type2 The type of the right-hand argument of the operator, if any. This option would be omitted
for a right-unary operator.
com_op The commutator of this operator.
neg_op The negator of this operator.
res_proc The restriction selectivity estimator function for this operator.
join_proc The join selectivity estimator function for this operator.
HASHES Indicates this operator can support a hash join.
left_sort_op If this operator can support a merge join, the operator that sorts the left-hand data
type of this operator.
right_sort_op If this operator can support a merge join, the operator that sorts the right-hand
data type of this operator.
Outputs
CREATE Message returned if the operator is successfully created.

Description
CREATE OPERATOR defines a new operator, name. The user who defines an operator becomes
its owner.
The operator name is a sequence of up to NAMEDATALEN-1 (31 by default) characters from
the following list:
+ - * / < > = ˜ ! @ # % ˆ & | ‘ ? $ :

There are a few restrictions on your choice of name:
• "$" and ":" cannot be defined as single-character operators, although they can be part of a
multi-character operator name.

APPENDIX D. REFERENCE MANUAL

294

• "–" and "/*" cannot appear anywhere in an operator name, since they will be taken as the start
of a comment.
• A multi-character operator name cannot end in "+" or "-", unless the name also contains at
least one of these characters:
˜ ! @ # % ˆ & | ‘ ? $ :

For example, @- is an allowed operator name, but *- is not. This restriction allows Postgres
to parse SQL-compliant queries without requiring spaces between tokens.
Note: When working with non-SQL-standard operator names, you will usually need to separate
adjacent operators with spaces to avoid ambiguity. For example, if you have defined a left-unary
operator named "@", you cannot write X*@Y; you must write X*@Y to ensure that Postgres reads
it as two operator names not one.
The operator "!=" is mapped to "<>" on input, so these two names are always equivalent.
At least one of LEFTARG and RIGHTARG must be defined. For binary operators, both should
be defined. For right unary operators, only LEFTARG should be defined, while for left unary
operators only RIGHTARG should be defined.
The func_name procedure must have been previously defined using CREATE FUNCTION
and must be defined to accept the correct number of arguments (either one or two) of the indicated
types.
The commutator operator should be identified if one exists, so that Postgres can reverse the
order of the operands if it wishes. For example, the operator area-less-than, <<<, would probably
have a commutator operator, area-greater-than, >>> . Hence, the query optimizer could freely
convert:
box ’((0,0), (1,1))’ >>> MYBOXES.description

to
MYBOXES.description <<< box ’((0,0), (1,1))’.
This allows the execution code to always use the latter representation and simplifies the query
optimizer somewhat.
Similarly, if there is a negator operator then it should be identified. Suppose that an operator,
area-equal, ===, exists, as well as an area not equal, !==. The negator link allows the query
optimizer to simplify
NOT MYBOXES.description === box ’((0,0), (1,1))’

to
MYBOXES.description !== box ’((0,0), (1,1))’.
If a commutator operator name is supplied, Postgres searches for it in the catalog. If it is found
and it does not yet have a commutator itself, then the commutator’s entry is updated to have the
newly created operator as its commutator. This applies to the negator, as well. This is to allow
the definition of two operators that are the commutators or the negators of each other. The first
operator should be defined without a commutator or negator (as appropriate). When the second

D.18. CREATE OPERATOR

295

operator is defined, name the first as the commutator or negator. The first will be updated as a
side effect. (As of Postgres 6.5, it also works to just have both operators refer to each other.)
The HASHES, SORT1, and SORT2 options are present to support the query optimizer in
performing joins. Postgres can always evaluate a join (i.e., processing a clause with two tuple
variables separated by an operator that returns a boolean) by iterative substitution [WONG76]. In
addition, Postgres can use a hash-join algorithm along the lines of [SHAP86]; however, it must
know whether this strategy is applicable. The current hash-join algorithm is only correct for
operators that represent equality tests; furthermore, equality of the data type must mean bitwise
equality of the representation of the type. (For example, a data type that contains unused bits that
don’t matter for equality tests could not be hashjoined.) The HASHES flag indicates to the query
optimizer that a hash join may safely be used with this operator.
Similarly, the two sort operators indicate to the query optimizer whether merge-sort is a usable
join strategy and which operators should be used to sort the two operand classes. Sort operators
should only be provided for an equality operator, and they should refer to less-than operators for
the left and right side data types respectively.
If other join strategies are found to be practical, Postgres will change the optimizer and runtime system to use them and will require additional specification when an operator is defined.
Fortunately, the research community invents new join strategies infrequently, and the added
generality of user-defined join strategies was not felt to be worth the complexity involved.
The RESTRICT and JOIN options assist the query optimizer in estimating result sizes. If a
clause of the form: MYBOXES.description <<< box ’((0,0), (1,1))’
is present in the qualification, then Postgres may have to estimate the fraction of the instances
in MYBOXES that satisfy the clause. The function res_proc must be a registered function (meaning
it is already defined using CREATE FUNCTION ) which accepts arguments of the correct data
types and returns a floating point number. The query optimizer simply calls this function, passing
the parameter ((0,0), (1,1)) and multiplies the result by the relation size to get the expected number
of instances.
Similarly, when the operands of the operator both contain instance variables, the query optimizer
must estimate the size of the resulting join. The function join_proc will return another floating
point number which will be multiplied by the cardinalities of the two classes involved to compute
the expected result size.
The difference between the function
my_procedure_1 (MYBOXES.description, box ’((0,0), (1,1))’)

and the operator
MYBOXES.description === box ’((0,0), (1,1))’

is that Postgres attempts to optimize operators and can decide to use an index to restrict the
search space when operators are involved. However, there is no attempt to optimize functions,
and they are performed by brute force. Moreover, functions can have any number of arguments
while operators are restricted to one or two.

APPENDIX D. REFERENCE MANUAL

296
Notes

Refer to the chapter on operators in the PostgreSQL User’s Guide for further information. Refer to
DROP OPERATOR to delete user-defined operators from a database.

Compatibility
SQL92
CREATE OPERATOR is a Postgres extension. There is no CREATE OPERATOR statement
in SQL92.

D.19

CREATE RULE

Name
CREATE RULE — Defines a new rule
CREATE RULE name AS ON event
TO object
[ WHERE condition ]
DO [ INSTEAD ] [ action | NOTHING ]
Inputs
name The name of a rule to create.
event Event is one of select, update, delete or insert.

D.19. CREATE RULE

297

object Object is either table or table.column.
condition Any SQL WHERE clause, new or old, can appear instead of an instance variable
whenever an instance variable is permissible in SQL.
action Any SQL statement, new or old, can appear instead of an instance variable whenever an
instance variable is permissible in SQL.
Outputs
CREATE Message returned if the rule is successfully created.

Description
The Postgres rule system allows one to define an alternate action to be performed on inserts,
updates, or deletions from database tables or classes. Currently, rules are used to implement table
views.
The semantics of a rule is that at the time an individual instance is accessed, inserted, updated,
or deleted, there is an old instance (for selects, updates and deletes) and a new instance (for inserts
and updates). If the event specified in the ON clause and the condition specified in the WHERE
clause are true for the old instance, the action part of the rule is executed. First, however, values
from fields in the old instance and/or the new instance are substituted for old.attribute-name and
new.attribute-name.
The action part of the rule executes with the same command and transaction identifier as the
user command that caused activation.
Notes
A caution about SQL rules is in order. If the same class name or instance variable appears in the
event, condition and action parts of a rule, they are all considered different tuple variables. More
accurately, new and old are the only tuple variables that are shared between these clauses. For
example, the following two rules have the same semantics:
ON UPDATE TO emp.salary
WHERE emp.name = "Joe" DO
UPDATE emp SET ...
WHERE ...
ON UPDATE TO emp-1.salary
WHERE emp-2.name = "Joe" DO
UPDATE emp-3 SET ...
WHERE ...

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298

Each rule can have the optional tag INSTEAD. Without this tag, action will be performed in addition
to the user command when the event in the condition part of the rule occurs. Alternately, the action
part will be done instead of the user command. In this latter case, the action can be the keyword
NOTHING.
It is very important to note to avoid circular rules. For example, though each of the following
two rule definitions are accepted by Postgres, the select command will cause Postgres to report
an error because the query cycled too many times:
Example of a circular rewrite rule combination.
CREATE RULE bad_rule_combination_1 AS
ON SELECT TO emp DO INSTEAD
SELECT TO toyemp;
CREATE RULE bad_rule_combination_2 AS
ON SELECT TO toyemp DO INSTEAD
SELECT TO emp;

This attempt to select from EMP will cause Postgres to issue an error because the queries cycled
too many times:
SELECT * FROM emp;

You must have rule definition access to a class in order to define a rule on it. Use GRANT and
REVOKE to change permissions.
The object in a SQL rule cannot be an array reference and cannot have parameters.
Aside from the "oid" field, system attributes cannot be referenced anywhere in a rule. Among
other things, this means that functions of instances (e.g., foo(emp) where emp is a class) cannot
be called anywhere in a rule.
The rule system stores the rule text and query plans as text attributes. This implies that
creation of rules may fail if the rule plus its various internal representations exceed some value
that is on the order of one page (8KB).

Usage
Make Sam get the same salary adjustment as Joe:
CREATE RULE example_1 AS
ON UPDATE emp.salary
WHERE old.name = "Joe" DO
UPDATE emp SET salary = new.salary
WHERE emp.name = "Sam";

D.19. CREATE RULE

299

At the time Joe receives a salary adjustment, the event will become true and Joe’s old instance and
proposed new instance are available to the execution routines. Hence, his new salary is substituted
into the action part of the rule which is subsequently executed. This propagates Joe’s salary on to
Sam.
Make Bill get Joe’s salary when it is accessed:
CREATE RULE example_2 AS
ON SELECT TO EMP.salary
WHERE old.name = "Bill" DO INSTEAD
SELECT emp.salary
FROM emp
WHERE emp.name = "Joe";

Deny Joe access to the salary of employees in the shoe department (current_user returns the
name of the current user):
CREATE RULE example_3 AS
ON SELECT TO emp.salary
WHERE old.dept = "shoe" AND current_user = "Joe" DO INSTEAD
NOTHING;

Create a view of the employees working in the toy department:
CREATE toyemp(name = char16, salary = int4);
CREATE RULE example_4 AS
ON SELECT TO toyemp DO INSTEAD
SELECT emp.name, emp.salary
FROM emp
WHERE emp.dept = "toy";

All new employees must make 5,000 or less:
CREATE RULE example_5 AS
ON INERT TO emp
WHERE new.salary > 5000 DO
UPDATE emp SET salary = 5000
WHERE emp.oid = new.oid;

Compatibility
SQL92
CREATE RULE statement is a Postgres language extension. There is no CREATE RULE
statement in SQL92.

APPENDIX D. REFERENCE MANUAL

300

D.20

CREATE SEQUENCE

Name
CREATE SEQUENCE â&#x20AC;&#x201D; Creates a new sequence number generator
CREATE SEQUENCE seqname
[ INCREMENT increment ]
[ MINVALUE minvalue ]
[ MAXVALUE maxvalue ]
[ START start ]
[ CACHE cache ]
[ CYCLE ]
Inputs
seqname The name of a sequence to be created.
increment The INCREMENT increment clause is optional. A positive value will make an ascending sequence, a negative one a descending sequence. The default value is one (1).
minvalue The optional clause MINVALUE minvalue determines the minimum value a sequence
can generate. The defaults are 1 and -2147483647 for ascending and descending sequences,
respectively.
maxvalue Use the optional clause MAXVALUE maxvalue to determine the maximum value for
the sequence. The defaults are 2147483647 and -1 for ascending and descending sequences,
respectively.
start The optional START start clause enables the sequence to begin anywhere. The default
starting value is minvalue for ascending sequences and maxvalue for descending ones.
cache The CACHE cache option enables sequence numbers to be preallocated and stored in
memory for faster access. The minimum value is 1 (only one value can be generated at a
time, i.e., no cache) and this is also the default.
CYCLE The optional CYCLE keyword may be used to enable the sequence to continue when the
maxvalue or minvalue has been reached by an ascending or descending sequence respectively.
If the limit is reached, the next number generated will be whatever the minvalue or maxvalue
is, as appropriate.
Outputs
CREATE Message returned if the command is successful.

Description
CREATE SEQUENCE will enter a new sequence number generator into the current data base.
This involves creating and initializing a new single-row table with the name seqname. The generator
will be "owned" by the user issuing the command.
After a sequence is created, you may use the function nextval(seqname ) to get a new number
from the sequence. The function currval(’seqname ’) may be used to determine the number
returned by the last call to nextval(seqname ) for the specified sequence in the current session.
The function setval(’seqname ’, newvalue ) may be used to set the current value of the specified
sequence. The next call to nextval(seqname ) will return the given value plus the sequence
increment.
Use a query like SELECT * FROM seqname; to get the parameters of a sequence. As an alternative
to fetching the parameters from the original definition as above, you can use SELECT last_value
FROM seqname; to obtain the last value allocated by any backend.
Low-level locking is used to enable multiple simultaneous calls to a generator.
Notes
Use DROP SEQUENCE to remove a sequence.
Each backend uses its own cache to store allocated numbers. Numbers that are cached but not
used in the current session will be lost, resulting in "holes" in the sequence.

table The name of a new class or table to be created.
column The name of a column.
type The type of the column. This may include array specifiers. Refer to the PostgreSQL User’s
Guide for further information about data types and arrays.
DEFAULT value A default value for a column. See the DEFAULT clause for more information.
column_constraint_clause The optional column constraint clauses specify a list of integrity
constraints or tests which new or updated entries must satisfy for an insert or update
operation to succeed. Each constraint must evaluate to a boolean expression. Although
SQL92 requires the column_constraint_clause to refer to that column only, Postgres allows
multiple columns to be referenced within a single column constraint. See the column
constraint clause for more information.
table_constraint_clause The optional table CONSTRAINT clause specifies a list of integrity
constraints which new or updated entries must satisfy for an insert or update operation to
succeed. Each constraint must evaluate to a boolean expression. Multiple columns may be
referenced within a single constraint. Only one PRIMARY KEY clause may be specified for a
table; PRIMARY KEY column (a table constraint) and PRIMARY KEY (a column constraint)
are mutually exclusive. See the table constraint clause for more information.
INHERITS inherited_table The optional INHERITS clause specifies a collection of table names
from which this table automatically inherits all fields. If any inherited field name appears
more than once, Postgres reports an error. Postgres automatically allows the created table
to inherit functions on tables above it in the inheritance hierarchy.
Outputs
CREATE Message returned if table is successfully created.
ERROR Message returned if table creation failed. This is usually accompanied by some descriptive text, such as: ERROR: Relation ’table ’ already exists, which occurs at runtime if the
table specified already exists in the database.
ERROR: DEFAULT: type mismatched If data type of default value doesn’t match the column
definition’s data type.

Description
CREATE TABLE will enter a new class or table into the current data base. The table will be
"owned" by the user issuing the command.

APPENDIX D. REFERENCE MANUAL

304

Each type may be a simple type, a complex type (set) or an array type. Each attribute may be
specified to be non-null and each may have a default value, specified by the DEFAULT Clause.
Note: Consistent array dimensions within an attribute are not enforced. This will likely change
in a future release.
The optional INHERITS clause specifies a collection of class names from which this class
automatically inherits all fields. If any inherited field name appears more than once, Postgres
reports an error. Postgres automatically allows the created class to inherit functions on classes
above it in the inheritance hierarchy. Inheritance of functions is done according to the conventions
of the Common Lisp Object System (CLOS).
Each new table or class table is automatically created as a type. Therefore, one or more
instances from the class are automatically a type and can be used in ALTER TABLE or other
CREATE TABLE statements.
The new table is created as a heap with no initial data. A table can have no more than 1600
columns (realistically, this is limited by the fact that tuple sizes must be less than 8192 bytes), but
this limit may be configured lower at some sites. A table cannot have the same name as a system
catalog table.

DEFAULT Clause
DEFAULT value
Inputs
value The possible values for the default value expression are:
• a literal value
• a user function
• a niladic function
Outputs
None.
Description
The DEFAULT clause assigns a default data value to a column (via a column definition in the
CREATE TABLE statement). The data type of a default value must match the column definition’s
data type.
An INSERT operation that includes a column without a specified default value will assign the
NULL value to the column if no explicit data value is provided for it. Default literal means that

D.21. CREATE TABLE

305

the default is the specified constant value. Default niladic-function or user-function means that the
default is the value of the specified function at the time of the INSERT.
There are two types of niladic functions:
niladic USER
CURRENT_USER / USER See CURRENT_USER function
SESSION_USER See CURRENT_USER function
SYSTEM_USER Not implemented
niladic datetime
CURRENT_DATE See CURRENT_DATE function
CURRENT_TIME See CURRENT_TIME function
CURRENT_TIMESTAMP See CURRENT_TIMESTAMP function
Usage
To assign a constant value as the default for the columns did and number, and a string literal to the
column did:
CREATE TABLE video_sales (
did VARCHAR(40) DEFAULT ’luso films’,
number INTEGER DEFAULT 0,
total CASH DEFAULT ’$0.0’
);

To assign an existing sequence as the default for the column did, and a literal to the column name:
CREATE TABLE distributors (
did DECIMAL(3) DEFAULT NEXTVAL(’serial’),
name VARCHAR(40) DEFAULT ’luso films’
);

name An arbitrary name given to the integrity constraint. If name is not specified, it is generated
from the table and column names, which should ensure uniqueness for name.
NULL The column is allowed to contain NULL values. This is the default.
NOT NULL The column is not allowed to contain NULL values. This is equivalent to the column
constraint CHECK (column NOT NULL).
UNIQUE The column must have unique values. In Postgres this is enforced by an implicit
creation of a unique index on the table.
PRIMARY KEY This column is a primary key, which implies that uniqueness is enforced by the
system and that other tables may rely on this column as a unique identifier for rows. See
PRIMARY KEY for more information.
constraint The definition of the constraint.
Description
The optional constraint clauses specify constraints or tests which new or updated entries must
satisfy for an insert or update operation to succeed. Each constraint must evaluate to a boolean
expression. Multiple attributes may be referenced within a single constraint. The use of PRIMARY
KEY as a table constraint is mutually incompatible with PRIMARY KEY as a column constraint.
A constraint is a named rule: an SQL object which helps define valid sets of values by putting
limits on the results of INSERT, UPDATE or DELETE operations performed on a Base Table.
There are two ways to define integrity constraints: table constraints, covered later, and column
constraints, covered here.
A column constraint is an integrity constraint defined as part of a column definition, and logically
becomes a table constraint as soon as it is created. The column constraints available are:
PRIMARY KEY
REFERENCES
UNIQUE
CHECK
NOT NULL

NOT NULL Constraint
[ CONSTRAINT name ] NOT NULL
The NOT NULL constraint specifies a rule that a column may contain only non-null values.
This is a column constraint only, and not allowed as a table constraint.

D.21. CREATE TABLE

307

Outputs
status ERROR: ExecAppend: Fail to add null value in not null attribute "column ". This error
occurs at runtime if one tries to insert a null value into a column which has a NOT NULL
constraint.

Description
Usage
Define two NOT NULL column constraints on the table distributors, one of which being a named
constraint:
CREATE TABLE distributors (
did DECIMAL(3) CONSTRAINT no_null NOT NULL,
name VARCHAR(40) NOT NULL
);

UNIQUE Constraint
[ CONSTRAINT name ] UNIQUE
Inputs
CONSTRAINT name An arbitrary label given to a constraint.
Outputs
status
ERROR: Cannot insert a duplicate key into a unique index. This error occurs at runtime if one tries to insert a duplicate value into a column.
Description
The UNIQUE constraint specifies a rule that a group of one or more distinct columns of a table
may contain only unique values.
The column definitions of the specified columns do not have to include a NOT NULL constraint
to be included in a UNIQUE constraint. Having more than one null value in a column without a
NOT NULL constraint, does not violate a UNIQUE constraint. (This deviates from the SQL92
definition, but is a more sensible convention. See the section on compatibility for more details.)
Each UNIQUE column constraint must name a column that is different from the set of columns
named by any other UNIQUE or PRIMARY KEY constraint defined for the table.

APPENDIX D. REFERENCE MANUAL

308

Note: Postgres automatically creates a unique index for each UNIQUE constraint, to assure
data integrity. See CREATE INDEX for more information.
Usage
Defines a UNIQUE column constraint for the table distributors. UNIQUE column constraints can
only be defined on one column of the table:
CREATE TABLE distributors ( did DECIMAL(3),
name VARCHAR(40) UNIQUE
);

which is equivalent to the following specified as a table constraint:
CREATE TABLE distributors ( did DECIMAL(3),
name VARCHAR(40),
UNIQUE(name)
);

The CHECK Constraint
[ CONSTRAINT name ] CHECK ( condition [, ...] )
Inputs
name An arbitrary name given to a constraint.
condition Any valid conditional expression evaluating to a boolean result.
Outputs
status
ERROR: ExecAppend: rejected due to CHECK constraint "table_column ". This error occurs at runtime if one tries to insert an illegal value into a column subject to a
CHECK constraint.
Description
The CHECK constraint specifies a restriction on allowed values within a column. The CHECK
constraint is also allowed as a table constraint.
The SQL92 CHECK column constraints can only be defined on, and refer to, one column of the
table. Postgres does not have this restriction.

D.21. CREATE TABLE

309

PRIMARY KEY Constraint
[ CONSTRAINT name ] PRIMARY KEY
Inputs
CONSTRAINT name An arbitrary name for the constraint.
Outputs
ERROR: Cannot insert a duplicate key into a unique index. This occurs at runtime if one
tries to insert a duplicate value into a column subject to a PRIMARY KEY constraint.
Description
The PRIMARY KEY column constraint specifies that a column of a table may contain only unique
(non-duplicate), non-NULL values. The definition of the specified column does not have to include
an explicit NOT NULL constraint to be included in a PRIMARY KEY constraint.
Only one PRIMARY KEY can be specified for a table.
Notes
Postgres automatically creates a unique index to assure data integrity (see CREATE INDEX
statement).
The PRIMARY KEY constraint should name a set of columns that is different from other
sets of columns named by any UNIQUE constraint defined for the same table, since it will result
in duplication of equivalent indexes and unproductive additional runtime overhead. However,
Postgres does not specifically disallow this.
REFERENCES Constraint
[ CONSTRAINT name ] REFERENCES reftable [ ( refcolumn ) ] [ MATCH matchtype ] [ ON
DELETE action ] [ ON UPDATE action ] [ [ NOT ] DEFERRABLE ] [ INITIALLY checktime ]
The REFERENCES constraint specifies a rule that a column value is checked against the
values of another column. REFERENCES can also be specified as part of a FOREIGN KEY table
constraint.
Inputs
CONSTRAINT name An arbitrary name for the constraint.
reftable The table that contains the data to check against.

APPENDIX D. REFERENCE MANUAL

310

refcolumn The column in reftable to check the data against. If this is not specified, the PRIMARY
KEY of the reftable is used.
MATCH matchtype There are three match types: MATCH FULL, MATCH PARTIAL, and
a default match type if none is specified. MATCH FULL will not allow one column of a
multi-column foreign key to be NULL unless all foreign key columns are NULL. The default
MATCH type allows some foreign key columns to be NULL while other parts of the foreign
key are not NULL. MATCH PARTIAL is currently not supported.
ON DELETE action The action to do when a referenced row in the referenced table is being
deleted. There are the following actions.
NO ACTION Produce error if foreign key violated. This is the default.
RESTRICT Same as NO ACTION.
CASCADE Delete any rows referencing the deleted row.
SET NULL Set the referencing column values to NULL.
SET DEFAULT Set the referencing column values to their default value.
ON UPDATE action The action to do when a referenced column in the referenced table is being
updated to a new value. If the row is updated, but the referenced column is not changed, no
action is done. There are the following actions.
NO ACTION Produce error if foreign key violated. This is the default.
RESTRICT Same as NO ACTION.
CASCADE Update the value of the referencing column to the new value of the referenced
column.
SET NULL Set the referencing column values to NULL.
SET DEFAULT Set the referencing column values to their default value.
[ NOT DEFERRABLE ] This controls whether the constraint can be deferred to the end of
the transaction. If DEFERRABLE, SET CONSTRAINTS ALL DEFERRED will cause the
foreign key to be checked only at the end of the transaction. NOT DEFERRABLE is the
default.
INITIALLY checktime checktime has two possible values which specify the default time to check
the constraint.
DEFERRED Check constraint only at the end of the transaction.
IMMEDIATE Check constraint after each statement. This is the default.
Outputs

D.21. CREATE TABLE

311

status
ERROR: name referential integrity violation - key referenced from table not found in
reftable This error occurs at runtime if one tries to insert a value into a column which does
not have a matching column in the referenced table.
Description
The REFERENCES column constraint specifies that a column of a table must only contain values
which match against values in a referenced column of a referenced table.
A value added to this column is matched against the values of the referenced table and referenced
column using the given match type. In addition, when the referenced column data is changed,
actions are run upon this columnâ&#x20AC;&#x2122;s matching data.
Notes
Currently Postgres only supports MATCH FULL and a default match type. In addition, the
referenced columns are supposed to be the columns of a UNIQUE constraint in the referenced
table, however Postgres does not enforce this.

A table constraint is an integrity constraint defined on one or more columns of a base table. The
four variations of "Table Constraint" are:
UNIQUE
CHECK
PRIMARY KEY
FOREIGN KEY

UNIQUE Constraint
[ CONSTRAINT name ] UNIQUE ( column [, ...] )
Inputs
CONSTRAINT name An arbitrary name given to a constraint.
column A name of a column in a table.
Outputs
status
ERROR: Cannot insert a duplicate key into a unique index This error occurs at runtime if one tries to insert a duplicate value into a column.
Description
The UNIQUE constraint specifies a rule that a group of one or more distinct columns of a table
may contain only unique values. The behavior of the UNIQUE table constraint is the same as that
for column constraints, with the additional capability to span multiple columns.
See the section on the UNIQUE column constraint for more details.
Usage
Define a UNIQUE table constraint for the table distributors:
CREATE TABLE distributors (
did DECIMAL(3),
name VARCHAR(40),
UNIQUE(name)
);

D.21. CREATE TABLE

313

PRIMARY KEY Constraint
[ CONSTRAINT name ] PRIMARY KEY ( column [, ...] )
Inputs
CONSTRAINT name An arbitrary name for the constraint.
column [, ... ] The names of one or more columns in the table.
Outputs
status
ERROR: Cannot insert a duplicate key into a unique index. This occurs at run-time if
one tries to insert a duplicate value into a column subject to a PRIMARY KEY constraint.
Description
The PRIMARY KEY constraint specifies a rule that a group of one or more distinct columns of
a table may contain only unique (nonduplicate), non-null values. The column definitions of the
specified columns do not have to include a NOT NULL constraint to be included in a PRIMARY
KEY constraint.
The PRIMARY KEY table constraint is similar to that for column constraints, with the additional
capability of encompassing multiple columns.
Refer to the section on the PRIMARY KEY column constraint for more information.
REFERENCES Constraint
[ CONSTRAINT name ] FOREIGN KEY ( column [, ...] ) REFERENCES reftable [ ( refcolumn [, ...]
) ] [ MATCH matchtype ] [ ON DELETE action ] [ ON UPDATE action ] [ [ NOT ] DEFERRABLE
] [ INITIALLY checktime ]
The REFERENCES constraint specifies a rule that a column value is checked against the
values of another column. REFERENCES can also be specified as part of a FOREIGN KEY table
constraint.
Inputs
CONSTRAINT name An arbitrary name for the constraint.
column [, ... ] The names of one or more columns in the table.
reftable The table that contains the data to check against.

APPENDIX D. REFERENCE MANUAL

314

referenced column [, ... ] One or more column in the reftable to check the data against. If this is
not specified, the PRIMARY KEY of the reftable is used.
MATCH matchtype There are three match types: MATCH FULL, MATCH PARTIAL, and
a default match type if none is specified. MATCH FULL will not allow one column of a
multi-column foreign key to be NULL unless all foreign key columns are NULL. The default
MATCH type allows a some foreign key columns to be NULL while other parts of the foreign
key are not NULL. MATCH PARTIAL is currently not supported.
ON DELETE action The action to do when a referenced row in the referenced table is being
deleted. There are the following actions.
NO ACTION Produce error if foreign key violated. This is the default.
RESTRICT Same as NO ACTION.
CASCADE Delete any rows referencing the deleted row.
SET NULL Set the referencing column values to NULL.
SET DEFAULT Set the referencing column values to their default value.
ON UPDATE action The action to do when a referenced column in the referenced table is being
updated to a new value. If the row is updated, but the referenced column is not changed, no
action is done. There are the following actions.
NO ACTION Produce error if foreign key violated. This is the default.
RESTRICT Disallow update of row being referenced.
CASCADE Update the value of the referencing column to the new value of the referenced
column.
SET NULL Set the referencing column values to NULL.
SET DEFAULT Set the referencing column values to their default value.
[ NOT DEFERRABLE ] This controls whether the constraint can be deferred to the end of
the transaction. If DEFERRABLE, SET CONSTRAINTS ALL DEFERRED will cause the
foreign key to be checked only at the end of the transaction. NOT DEFERRABLE is the
default.
INITIALLY checktime checktime has two possible values which specify the default time to
check the constraint.
IMMEDIATE Check constraint after each statement. This is the default.
DEFERRED Check constraint only at the end of the transaction.

D.21. CREATE TABLE

315

Outputs
status
ERROR: name referential integrity violation — key referenced from table not
found in reftable This error occurs at runtime if one tries to insert a value into a
column which does not have a matching column in the referenced table.
Description
The FOREIGN KEY constraint specifies a rule that a group of one or more distinct columns of a
table is related to a group of distinct columns in the referenced table.
The FOREIGN KEY table constraint is similar to that for column constraints, with the additional
capability of encompassing multiple columns.
Refer to the section on the FOREIGN KEY column constraint for more information.

NULL clause
The NULL "constraint" (actually a non-constraint) is a Postgres extension to SQL92 is included
for symmetry with the NOT NULL clause. Since it is the default for any column, its presence is
simply noise. [ CONSTRAINT name ] NULL
NOT NULL clause
SQL92 specifies some additional capabilities for NOT NULL:
[ CONSTRAINT name ] NOT NULL [ { INITIALLY DEFERRED | INITIALLY IMMEDIATE} ] [ [ NOT ] DEFERRABLE ]

CONSTRAINT clause
SQL92 specifies some additional capabilities for constraints, and also defines assertions and
domain constraints. Note: Postgres does not yet support either domains or assertions.
An assertion is a special type of integrity constraint and shares the same namespace as other
constraints. However, an assertion is not necessarily dependent on one particular base table as
constraints are, so SQL-92 provides the CREATE ASSERTION statement as an alternate method
for defining a constraint:
CREATE ASSERTION name CHECK ( condition )

A CONSTRAINT definition may contain one deferment attribute clause and/or one initial constraint
mode clause, in any order.

D.22. CREATE TABLE AS

319

NOT DEFERRABLE The constraint must be checked at the end of each statement. SET
CONSTRAINTS ALL DEFERRED will have no effect on this type of constraint.
DEFERRABLE This controls whether the constraint can be deferred to the end of the transaction. If SET CONSTRAINTS ALL DEFERRED is used or the constraint is set to INITIALLY
DEFERRED, this will cause the foreign key to be checked only at the end of the transaction.
SET CONSTRAINT changes the foreign key constraint mode only for the current transaction.
INITIALLY IMMEDIATE Check constraint only at the end of the transaction. This is the
default
INITIALLY DEFERRED Check constraint after each statement.
CHECK clause
SQL92 specifies some additional capabilities for CHECK in either table or column constraints.
table constraint definition:
[ CONSTRAINT name ] CHECK ( VALUE condition ) [ { INITIALLY DEFERRED | INITIALLY IMMEDIATE} ] [ [ NOT ] DEFERRABLE ]

column The name of a column. Multiple column names can be specified using a comma-delimited
list of column names.
select_clause A valid query statement. Refer to SELECT for a description of the allowed syntax.
Outputs
Refer to CREATE TABLE and SELECT for a summary of possible output messages.

Description
CREATE TABLE AS enables a table to be created from the contents of an existing table. It is
functionality equivalent to SELECT INTO, but with perhaps a more direct syntax.

D.23

CREATE TRIGGER

Name
CREATE TRIGGER â&#x20AC;&#x201D; Creates a new trigger
CREATE TRIGGER name { BEFORE | AFTER } { event [OR ...] }
ON table
FOR EACH { ROW | STATEMENT }
EXECUTE PROCEDURE func ( arguments )
Inputs
name The name of an existing trigger.
table The name of a table.
event One of INSERT, DELETE or UPDATE.
funcname A user-supplied function.
Outputs
CREATE This message is returned if the trigger is successfully created.

D.23. CREATE TRIGGER

321

Description
CREATE TRIGGER will enter a new trigger into the current data base. The trigger will be
associated with the relation relname and will execute the specified function funcname.
The trigger can be specified to fire either before BEFORE the operation is attempted on a
tuple (before constraints are checked and the INSERT, UPDATE or DELETE is attempted) or
AFTER the operation has been attempted (e.g., after constraints are checked and the INSERT,
UPDATE or DELETE has completed). If the trigger fires before the event, the trigger may skip
the operation for the current tuple, or change the tuple being inserted (for INSERT and UPDATE
operations only). If the trigger fires after the event, all changes, including the last insertion,
update, or deletion, are "visible" to the trigger.
Refer to the chapters on SPI and Triggers in the PostgreSQL Programmer’s Guide for more
information.
Notes
CREATE TRIGGER is a Postgres language extension.
Only the relation owner may create a trigger on this relation.
As of the current release (v7.0), STATEMENT triggers are not implemented.
Refer to DROP TRIGGER for information on how to remove triggers.

Usage
Check if the specified distributor code exists in the distributors table before appending or updating
a row in the table films:
CREATE TRIGGER if_dist_exists
BEFORE INSERT OR UPDATE ON films
FOR EACH ROW
EXECUTE PROCEDURE check_primary_key (’did’, ’distributors’, ’did’);

Before cancelling a distributor or updating its code, remove every reference to the table films:
CREATE TRIGGER if_film_exists
BEFORE DELETE OR UPDATE ON distributors
FOR EACH ROW
EXECUTE PROCEDURE check_foreign_key (1, ’CASCADE’, ’did’, ’films’, ’did’);

Compatibility
SQL92
There is no CREATE TRIGGER in SQL92.
The second example above may also be done by using a FOREIGN KEY constraint as in:

Name
CREATE TYPE — Defines a new base data type
CREATE TYPE typename (
INPUT = input_function ,
OUTPUT = output_function,
INTERNALLENGTH = { internallength | VARIABLE }
[ , EXTERNALLENGTH = { externallength | VARIABLE } ]
[ , DEFAULT = "default " ]
[ , ELEMENT = element ]
[ , DELIMITER = delimiter ]
[ , SEND = send_function ]
[ , RECEIVE = receive_function ]
[ , PASSEDBYVALUE ] )
Inputs
typename The name of a type to be created.
internallength A literal value, which specifies the internal length of the new type.
externallength A literal value, which specifies the external length of the new type.
input_function The name of a function, created by CREATE FUNCTION, which converts data
from its external form to the type’s internal form.
output_function The name of a function, created by CREATE FUNCTION, which converts
data from its internal form to a form suitable for display.
element The type being created is an array; this specifies the type of the array elements.
delimiter The delimiter character for the array.
default The default text to be displayed to indicate "data not present."

D.24. CREATE TYPE

323

send_function The name of a function, created by CREATE FUNCTION, which converts data
of this type into a form suitable for transmission to another machine.
receive_function The name of a function, created by CREATE FUNCTION, which converts
data of this type from a form suitable for transmission from another machine to internal form.
Outputs
CREATE Message returned if the type is successfully created.

Description
CREATE TYPE allows the user to register a new user data type with Postgres for use in the
current data base. The user who defines a type becomes its owner. typename is the name of the
new type and must be unique within the types defined for this database.
CREATE TYPE requires the registration of two functions (using create function) before
defining the type. The representation of a new base type is determined by input_function, which
converts the typeâ&#x20AC;&#x2122;s external representation to an internal representation usable by the operators
and functions defined for the type. Naturally, output_function performs the reverse transformation.
Both the input and output functions must be declared to take one or two arguments of type "opaque".
New base data types can be fixed length, in which case internallength is a positive integer, or
variable length, in which case Postgres assumes that the new type has the same format as the
Postgres-supplied data type, "text". To indicate that a type is variable length, set internallength to
VARIABLE. The external representation is similarly specified using the externallength keyword.
To indicate that a type is an array and to indicate that a type has array elements, indicate the
type of the array element using the element keyword. For example, to define an array of 4-byte
integers ("int4"), specify ELEMENT = int4
To indicate the delimiter to be used on arrays of this type, delimiter can be set to a specific
character. The default delimiter is the comma (",").
A default value is optionally available in case a user wants some specific bit pattern to mean
"data not present." Specify the default with the DEFAULT keyword.
The optional arguments send_function and receive_function are used when the application program requesting Postgres services resides on a different machine. In this case, the machine on
which Postgres runs may use a format for the data type different from that used on the remote
machine. In this case it is appropriate to convert data items to a standard form when sending from
the server to the client and converting from the standard format to the machine specific format
when the server receives the data from the client. If these functions are not specified, then it is
assumed that the internal format of the type is acceptable on all relevant machine architectures.
For example, single characters do not have to be converted if passed from a Sun-4 to a DECstation,
but many other types do.

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324

The optional flag, PASSEDBYVALUE, indicates that operators and functions which use this
data type should be passed an argument by value rather than by reference. Note that you may not
pass by value types whose internal representation is more than four bytes.
For new base types, a user can define operators, functions and aggregates using the appropriate
facilities described in this section.
Array Types
Two generalized built-in functions, array_in and array_out, exist for quick creation of variable
length array types. These functions operate on arrays of any existing Postgres type.
Large Object Types
A "regular" Postgres type can only be 8192 bytes in length. If you need a larger type you must
create a Large Object type. The interface for these types is discussed at length in the PostgreSQL
Programmerâ&#x20AC;&#x2122;s Guide. The length of all large object types is always VARIABLE.

Notes
Type names cannot begin with the underscore character ("_") and can only be 31 characters long.
This is because Postgres silently creates an array type for each base type with a name consisting
of the base type’s name prepended with an underscore.
Refer to DROP TYPE to remove an existing type.
See also CREATE FUNCTION, CREATE OPERATOR and the chapter on Large Objects
in the PostgreSQL Programmer’s Guide.

Compatibility
SQL3
CREATE TYPE is an SQL3 statement.

D.25

CREATE USER

Name
CREATE USER — Creates a new database user
CREATE USER username
[ WITH [ SYSID uid ]
[ PASSWORD ’password ’ ] ]
[ CREATEDB | NOCREATEDB ]
[ CREATEUSER | NOCREATEUSER ]
[ IN GROUP groupname [, ...] ]
[ VALID UNTIL ’abstime ’ ]
Inputs
username The name of the user.
uid The SYSID clause can be used to choose the Postgres user id of the user that is being created.
It is not at all necessary that those match the UNIX user ids, but some people choose to keep
the numbers the same.
If this is not specified, the highest assigned user id plus one will be used as default.
password Sets the user’s password. If you do not plan to use password authentication you can omit
this option, otherwise the user won’t be able to connect to a password-authenticated server.
See pg_hba.conf(5) or the administrator’s guide for details on how to set up authentication
mechanisms.

APPENDIX D. REFERENCE MANUAL

326

CREATEDB, NOCREATEDB These clauses define a user’s ability to create databases. If
CREATEDB is specified, the user being defined will be allowed to create his own databases.
Using NOCREATEDB will deny a user the ability to create databases. If this clause is
omitted, NOCREATEDB is used by default.
CREATEUSER, NOCREATEUSER These clauses determine whether a user will be permitted
to create new users himself. This option will also make the user a superuser who can override
all access restrictions. Omitting this clause will set the user’s value of this attribute to be
NOCREATEUSER.
groupname A name of a group into which to insert the user as a new member.
abstime The VALID UNTIL clause sets an absolute time after which the user’s password is no
longer valid. If this clause is omitted the login will be valid for all time.
Outputs
CREATE USER Message returned if the command completes successfully.

Description
CREATE USER will add a new user to an instance of Postgres. Refer to the administrator’s guide
for information about managing users and authentication. You must be a database superuser to
use this command.
Use ALTER USER to change a user’s password and privileges, and DROP USER to remove a
user. Use ALTER GROUP to add or remove the user from other groups. Postgres comes with a
script createuser which has the same functionality as this command (in fact, it calls this command)
but can be run from the command shell.

Usage
Create a user with no password:
CREATE USER jonathan

Create a user with a password:
CREATE USER davide WITH PASSWORD ’jw8s0F4’

Create a user with a password, whose account is valid until the end of 2001. Note that after one
second has ticked in 2002, the account is not valid:
CREATE USER miriam WITH PASSWORD ’jw8s0F4’ VALID UNTIL ’Jan 1 2002’

D.26. CREATE VIEW

327

Create an account where the user can create databases:
CREATE USER manuel WITH PASSWORD ’jw8s0F4’
CREATEDB

Compatibility
SQL92
There is no CREATE USER statement in SQL92.

D.26

CREATE VIEW

Name
CREATE VIEW — Constructs a virtual table
CREATE VIEW view AS SELECT query
Inputs
view The name of a view to be created.
query An SQL query which will provide the columns and rows of the view.
Refer to the SELECT statement for more information about valid arguments.
Outputs
CREATE The message returned if the view is successfully created.
ERROR: Relation ’view ’ already exists This error occurs if the view specified already exists
in the database.
NOTICE create: attribute named "column " has an unknown type The view will be created
having a column with an unknown type if you do not specify it. For example, the following
command gives a warning:
CREATE VIEW vista AS SELECT ’Hello World’

Description
CREATE VIEW will define a view of a table or class. This view is not physically materialized.
Specifically, a query rewrite retrieve rule is automatically generated to support retrieve operations
on views.
Notes
Currently, views are read only.
Use the DROP VIEW statement to drop views.

Compatibility
SQL92
SQL92 specifies some additional capabilities for the CREATE VIEW statement:
CREATE VIEW view [ column [, ...] ] AS
SELECT expression [ AS colname ] [, ...]
FROM table
[ WHERE condition ]
[ WITH [ CASCADE | LOCAL ] CHECK OPTION ]
The optional clauses for the full SQL92 command are:
CHECK OPTION This option is to do with updatable views. All INSERTs and UPDATEs on the
view will be checked to ensure data satisfy the view-defining condition. If they do not, the
update will be rejected.

D.27. CREATEDB

329

LOCAL Check for integrity on this view.
CASCADE Check for integrity on this view and on any dependent view. CASCADE is assumed
if neither CASCADE nor LOCAL is specified.

D.27

createdb

Name
createdb — Create a new Postgres database
createdb [ options ] dbname [ description ]
Inputs
-h, –host host Specifies the hostname of the machine on which the postmaster is running.
-p, –port port Specifies the Internet TCP/IP port or local Unix domain socket file extension on
which the postmaster is listening for connections.
-U, –username username Username to connect as.
-W, –password Force password prompt.
-e, –echo Echo the queries that createdb generates and sends to the backend.
-q, –quiet Do not display a response.
-D, –location datadir Specifies the alternate database location for this database installation. This
is the location of the installation system tables, not the location of this specific database, which
may be different.
-E, –encoding encoding Specifies the character encoding scheme to be used with this database.
dbname Specifies the name of the database to be created. The name must be unique among all
Postgres databases in this installation. The default is to create a database with the same
name as the current system user.
description This optionally specifies a comment to be associated with the newly created database.
The options -h, -p, -U, -W, and -e are passed on literally to psql.

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330
Outputs

CREATE DATABASE The database was successfully created.
createdb: Database creation failed. (Says it all.)
createdb: Comment creation failed. (Database was created.) The comment/description for
the database could not be created. the database itself will have been created already. You can
use the SQL command COMMENT ON DATABASE to create the comment later on.
If there is an error condition, the backend error message will be displayed. See CREATE
DATABASE and psql for possibilities.

Description
createdb creates a new Postgres database. The user who executes this command becomes the
database owner.
createdb is a shell script wrapper around the SQL command CREATE DATABASE via the
Postgres interactive terminal psql. Thus, there is nothing special about creating databases via this
or other methods. This means that the psql must be found by the script and that a database server
is running at the targeted host. Also, any default settings and environment variables available to
psql and the libpq front-end library do apply.

CREATE DATABASE The response is the same as you would have gotten from running the
CREATE DATABASE SQL command.
To create the database demo using the postmaster on host eden, port 5000, using the LATIN1
encoding scheme with a look at the underlying query:
$ createdb -p 5000 -h eden -E LATIN1 -e demo

or
CREATE DATABASE "demo" WITH ENCODING = ’LATIN1’ CREATE DATABASE

D.28. CREATELANG

D.28

331

createlang

Name
createlang — Add a new programming language to a Postgres database
createlang [ connection options ] [ langname [ dbname ] ]
createlang [ connection options ] –list|-l [dbname]
Inputs
createlang accepts the following command line arguments:
langname Specifies the name of the backend programming language to be defined. createlang
will prompt for langname if it is not specified on the command line.
-d, –dbname dbname Specifies which database the language should be added.
-l, –list Shows a list of already installed languages in the target database (which must be specified).
createlang also accepts the following command line arguments for connection parameters:
-h, –host host Specifies the hostname of the machine on which the postmaster is running.
-p, –port port Specifies the Internet TCP/IP port or local Unix domain socket file extension on
which the postmaster is listening for connections.
-U, –username username Username to connect as.
-W, –password Force password prompt.
Outputs
Most error messages are self-explanatory. If not, run createlang with the –echo option and see
under the respective SQL command for details. Check also under psql for more possibilities.

Description
createlang is a utility for adding a new programming language to a Postgres database. createlang
currently accepts two languages, plsql and pltcl.
Although backend programming languages can be added directly using several SQL commands,
it is recommended to use createlang because it performs a number of checks and is much easier
to use. See CREATE LANGUAGE for more.

Notes
Use droplang to remove a language.

APPENDIX D. REFERENCE MANUAL

332

Usage
To install pltcl:
$ createlang pltcl

D.29

createuser

Name
createuser — Create a new Postgres user
createuser [ options ] [ username ]
Inputs
-h, –host host Specifies the hostname of the machine on which the postmaster is running.
-p, –port port Specifies the Internet TCP/IP port or local Unix domain socket file extension on
which the postmaster is listening for connections.
-e, –echo Echo the queries that createdb generates and sends to the backend.
-q, –quiet Do not display a response.
-d, –createdb Allows the new user to create databases.
-D, –no-createdb Forbids the new user to create databases.
-a, –adduser Allows the new user to create other users.
-A, –no-adduser Forbids the new user to create other users.
-P, –pwprompt If given, createuser will issue a prompt for the password of the new user. This is
not necessary if you do not plan on using password authentication.
-i, –sysid uid Allows you to pick a non-default user id for the new user. This is not necessary,
but some people like it.
username Specifies the name of the Postgres user to be created. This name must be unique
among all Postgres users.
You will be prompted for a name and other missing information if it is not specified on the command
line. The options -h, -p, and -e, are passed on literally to psql. The psql options -U and -W are
available as well, but their use can be confusing in this context.

D.30. DECLARE

333

Outputs
CREATE USER All is well.
createuser: creation of user "username " failed Something went wrong. The user was not
created.
If there is an error condition, the backend error message will be displayed. See CREATE USER
and psql for possibilities.

Description
createuser creates a new Postgres user. Only users with usesuper set in the pg_shadow class can
create new Postgres users.
createuser is a shell script wrapper around the SQL command CREATE USER via the Postgres
interactive terminal psql. Thus, there is nothing special about creating users via this or other
methods. This means that the psql must be found by the script and that a database server is
running at the targeted host. Also, any default settings and environment variables available to psql
and the libpq front-end library do apply.

Usage
To create a user joe on the default database server:
$ createuser joe
Is the new user allowed to create databases? (y/n) n
Shall the new user be allowed to create more new users? (y/n) n
CREATE USER

Inputs
cursorname The name of the cursor to be used in subsequent FETCH operations.
BINARY Causes the cursor to fetch data in binary rather than in text format.
INSENSITIVE SQL92 keyword indicating that data retrieved from the cursor should be unaffected by updates from other processes or cursors. Since cursor operations occur within
transactions in Postgres this is always the case. This keyword has no effect.
SCROLL SQL92 keyword indicating that data may be retrieved in multiple rows per FETCH
operation. Since this is allowed at all times by Postgres this keyword has no effect.
query An SQL query which will provide the rows to be governed by the cursor. Refer to the
SELECT statement for further information about valid arguments.
READ ONLY SQL92 keyword indicating that the cursor will be used in a read only mode. Since
this is the only cursor access mode available in Postgres this keyword has no effect.
UPDATE SQL92 keyword indicating that the cursor will be used to update tables. Since cursor
updates are not currently supported in Postgres this keyword provokes an informational
error message.
column Column(s) to be updated. Since cursor updates are not currently supported in Postgres
the UPDATE clause provokes an informational error message.
Outputs
SELECT The message returned if the SELECT is run successfully.
NOTICE BlankPortalAssignName: portal "cursorname " already exists This error occurs
if cursorname is already declared.
ERROR: Named portals may only be used in begin/end transaction blocks This error occurs if the cursor is not declared within a transaction block.

Description
DECLARE allows a user to create cursors, which can be used to retrieve a small number of rows
at a time out of a larger query. Cursors can return data either in text or in binary format using
FETCH.
Normal cursors return data in text format, either ASCII or another encoding scheme depending
on how the Postgres backend was built. Since data is stored natively in binary format, the system
must do a conversion to produce the text format. In addition, text formats are often larger in size
than the corresponding binary format. Once the information comes back in text form, the client

D.30. DECLARE

335

application may need to convert it to a binary format to manipulate it. BINARY cursors give you
back the data in the native binary representation.
As an example, if a query returns a value of one from an integer column, you would get a string
of ’1’ with a default cursor whereas with a binary cursor you would get a 4-byte value equal to
control-A (ˆA).
BINARY cursors should be used carefully. User applications such as psql are not aware of binary
cursors and expect data to come back in a text format.
String representation is architecture-neutral whereas binary representation can differ between
different machine architectures and Postgres does not resolve byte ordering or representation issues for
binary cursors. Therefore, if your client machine and server machine use different representations
(e.g., "big-endian" versus "little-endian"), you will probably not want your data returned in binary
format. However, binary cursors may be a little more efficient since there is less conversion
overhead in the server to client data transfer. Tip: If you intend to display the data in ASCII,
getting it back in ASCII will save you some effort on the client side.
Notes
Cursors are only available in transactions. Use to BEGIN, COMMIT and ROLLBACK to define a
transaction block.
In SQL92 cursors are only available in embedded SQL (ESQL) applications. The Postgres
backend does not implement an explicit OPEN cursor statement; a cursor is considered to be
open when it is declared. However, ecpg, the embedded SQL preprocessor for Postgres, supports
the SQL92 cursor conventions, including those involving DECLARE and OPEN statements.

Usage
To declare a cursor:
DECLARE liahona CURSOR FOR
SELECT * FROM films;

Compatibility
SQL92
SQL92 allows cursors only in embedded SQL and in modules. Postgres permits cursors to be
used interactively. SQL92 allows embedded or modular cursors to update database information.
All Postgres cursors are read only. The BINARY keyword is a Postgres extension.

APPENDIX D. REFERENCE MANUAL

336

D.31

DELETE

Name
DELETE — Removes rows from a table
DELETE FROM table [ WHERE condition ]
Inputs
table The name of an existing table.
condition This is an SQL selection query which returns the rows which are to be deleted.
Refer to the SELECT statement for further description of the WHERE clause.
Outputs
DELETE count Message returned if items are successfully deleted. The count is the number of
rows deleted.
If count is 0, no rows were deleted.

Description
DELETE removes rows which satisfy the WHERE clause from the specified table.
If the condition (WHERE clause) is absent, the effect is to delete all rows in the table. The
result is a valid, but empty table. Tip: TRUNCATE is a Postgres extension which provides a faster
mechanism to remove all rows from a table.
You must have write access to the table in order to modify it, as well as read access to any table
whose values are read in the condition.

Compatibility
SQL92
SQL92 allows a positioned DELETE statement:
DELETE FROM table WHERE CURRENT OF cursor

where cursor identifies an open cursor. Interactive cursors in Postgres are read-only.

D.32

DROP AGGREGATE

Name
DROP AGGREGATE — Removes the definition of an aggregate function
DROP AGGREGATE name type
Inputs
name The name of an existing aggregate function.
type The type of an existing aggregate function. (Refer to the PostgreSQL User’s Guide for further
information about data types).
Outputs
DROP Message returned if the command is successful.
NOTICE RemoveAggregate: aggregate ’agg ’ for ’type ’ does not exist This message occurs
if the aggregate function specified does not exist in the database.

Description
DROP AGGREGATE will remove all references to an existing aggregate definition. To execute
this command the current user must be the owner of the aggregate.

Compatibility
SQL92
There is no DROP AGGREGATE statement in SQL92; the statement is a Postgres language
extension.

D.33

DROP DATABASE

Name
DROP DATABASE — Removes an existing database
DROP DATABASE name
Inputs
name The name of an existing database to remove.
Outputs
DROP DATABASE This message is returned if the command is successful.
ERROR: user ’username ’ is not allowed to create/drop databases You must have the special CREATEDB privilege to drop databases. See CREATE USER.
ERROR: dropdb: cannot be executed on the template database The template1 database cannot be removed. It’s not in your interest.
ERROR: dropdb: cannot be executed on an open database You cannot be connected to the
database your are about to remove. Instead, you could connect to template1 or any other
database and run this command again.
ERROR: dropdb: database ’name ’ does not exist This message occurs if the specified database
does not exist.

D.34. DROP FUNCTION

339

ERROR: dropdb: database ’name ’ is not owned by you You must be the owner of the database.
Being the owner usually means that you created it as well.
ERROR: dropdb: May not be called in a transaction block. You must finish the transaction
in progress before you can call this command.
NOTICE: The database directory ’xxx’ could not be removed. The database was dropped (unless other error messages came up), but the directory where the data is stored could not be
removed. You must delete it manually.

Description
DROP DATABASE removes the catalog entries for an existing database and deletes the directory
containing the data. It can only be executed by the database owner (usually the user that created
it).
Notes
This command cannot be executed while connected to the target database. Thus, it might be more
convenient to use the shell script dropdb, which is a wrapper around this command, instead.
Refer to CREATE DATABASE for information on how to create a database.

Compatibility
SQL92
DROP DATABASE statement is a Postgres language extension; there is no such command in
SQL92.

D.34

DROP FUNCTION

Name
DROP FUNCTION — Removes a user-defined C function
DROP FUNCTION name ( [ type [, ...] ] )
Inputs
name The name of an existing function.
type The type of function parameters.

APPENDIX D. REFERENCE MANUAL

340
Outputs

DROP Message returned if the command completes successfully.
NOTICE RemoveFunction: Function "name " ("types ") does not exist This message is given
if the function specified does not exist in the current database.

Description
DROP FUNCTION will remove references to an existing C function. To execute this command
the user must be the owner of the function. The input argument types to the function must be
specified, as only the function with the given name and argument types will be removed.
Notes
Refer to CREATE FUNCTION for information on creating aggregate functions.
No checks are made to ensure that types, operators or access methods that rely on the function
have been removed first.

Inputs
name The name of an existing group.
Outputs
DROP GROUP The message returned if the group is successfully deleted.

Description
DROP GROUP removes the specified group from the database. The users in the group are not
deleted.
Use CREATE GROUP to add new groups, and ALTER GROUP to change a group’s membership.

Usage
To drop a group:
DROP GROUP staff;

Compatibility
SQL92
There is no DROP GROUP in SQL92.

D.36

DROP INDEX

Name
DROP INDEX — Removes an index from a database
DROP INDEX index_name
Inputs
index_name The name of the index to remove.
Outputs
DROP The message returned if the index is successfully dropped.
ERROR: index "index_name " nonexistent This message occurs if index_name is not an index
in the database.

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342

Description
DROP INDEX drops an existing index from the database system. To execute this command you
must be the owner of the index.
Notes
DROP INDEX is a Postgres language extension.
Refer to CREATE INDEX for information on how to create indexes.

Compatibility
SQL92
SQL92 defines commands by which to access a generic relational database. Indexes are an
implementation-dependent feature and hence there are no index-specific commands or definitions
in the SQL92 language.

D.37

DROP LANGUAGE

Name
DROP LANGUAGE — Removes a user-defined procedural language
DROP PROCEDURAL LANGUAGE ’name ’
Inputs
name The name of an existing procedural language.
Outputs
DROP This message is returned if the language is successfully dropped.
ERROR: Language "name " doesn’t exist This message occurs if a language called name is not
found in the database.

D.38. DROP OPERATOR

343

Description
DROP PROCEDURAL LANGUAGE will remove the definition of the previously registered
procedural language called name.
Notes
The DROP PROCEDURAL LANGUAGE statement is a Postgres language extension.
Refer to CREATE LANGUAGE for information on how to create procedural languages.
No checks are made if functions or trigger procedures registered in this language still exist.
To re-enable them without having to drop and recreate all the functions, the pg_proc’s prolang
attribute of the functions must be adjusted to the new object ID of the recreated pg_language entry
for the PL.

DROP The message returned if the command is successful.
ERROR: RemoveOperator: binary operator ’oper’ taking ’type’ and ’type2’ does not exist
This message occurs if the specified binary operator does not exist.
ERROR: RemoveOperator: left unary operator ’oper ’ taking ’type’ does not exist This message occurs if the left unary operator specified does not exist.
ERROR: RemoveOperator: right unary operator ’oper’ taking ’type’ does not exist This message occurs if the right unary operator specified does not exist.

Description
DROP OPERATOR drops an existing operator from the database. To execute this command you
must be the owner of the operator.
The left or right type of a left or right unary operator, respectively, may be specified as NONE.
Notes
The DROP OPERATOR statement is a Postgres language extension.
Refer to CREATE OPERATOR for information on how to create operators.
It is the user’s responsibility to remove any access methods and operator classes that rely on
the deleted operator.

Name
DROP RULE â&#x20AC;&#x201D; Removes an existing rule from the database
DROP RULE name
Inputs
name The name of an existing rule to drop.
Outputs
DROP Message returned if successful.
ERROR: RewriteGetRuleEventRel: rule "name" not found This message occurs if the specified rule does not exist.

Description
DROP RULE drops a rule from the specified Postgres rule system. Postgres will immediately
cease enforcing it and will purge its definition from the system catalogs.
Notes
The DROP RULE statement is a Postgres language extension.
Refer to CREATE RULE for information on how to create rules.
Once a rule is dropped, access to historical information the rule has written may disappear.

Usage
To drop the rewrite rule newrule:
DROP RULE newrule;

Compatibility
SQL92
There is no DROP RULE in SQL92.

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346

D.40

DROP SEQUENCE

Name
DROP SEQUENCE â&#x20AC;&#x201D; Removes an existing sequence
DROP SEQUENCE name [, ...]
Inputs
name The name of a sequence.
Outputs
DROP The message returned if the sequence is successfully dropped.
NOTICE: Relation "name" does not exist. This message occurs if the specified sequence does
not exist.

Description
DROP SEQUENCE removes sequence number generators from the data base. With the current
implementation of sequences as special tables it works just like the DROP TABLE statement.
Notes
The DROP SEQUENCE statement is a Postgres language extension.
Refer to the CREATE SEQUENCE statement for information on how to create a sequence.

Usage
To remove sequence serial from database:
DROP SEQUENCE serial;

Compatibility
SQL92
There is no DROP SEQUENCE in SQL92.

D.41. DROP TABLE

D.41

347

DROP TABLE

Name
DROP TABLE â&#x20AC;&#x201D; Removes existing tables from a database
DROP TABLE name [, ...]
Inputs
name The name of an existing table or view to drop.
Outputs
DROP The message returned if the command completes successfully.
ERROR Relation "name" Does Not Exist! If the specified table or view does not exist in the
database.

Description
DROP TABLE removes tables and views from the database. Only its owner may destroy a table
or view. A table may be emptied of rows, but not destroyed, by using DELETE.
If a table being destroyed has secondary indexes on it, they will be removed first. The removal
of just a secondary index will not affect the contents of the underlying table.
Notes
Refer to CREATE TABLE and ALTER TABLE for information on how to create or modify
tables.

RESTRICT Ensures that only a table with no dependent views or integrity constraints can be
destroyed.
CASCADE Any referencing views or integrity constraints will also be dropped.
Tip: At present, to remove a referenced view you must drop it explicitly.

D.42

DROP TRIGGER

Name
DROP TRIGGER â&#x20AC;&#x201D; Removes the definition of a trigger
DROP TRIGGER name ON table
Inputs
name The name of an existing trigger.
table The name of a table.
Outputs
DROP The message returned if the trigger is successfully dropped.
ERROR: DropTrigger: there is no trigger name on relation "table" This message occurs if
the trigger specified does not exist.

Description
DROP TRIGGER will remove all references to an existing trigger definition. To execute this
command the current user must be the owner of the trigger.
Notes
DROP TRIGGER is a Postgres language extension.
Refer to CREATE TRIGGER for information on how to create triggers.

Name
DROP TYPE — Removes a user-defined type from the system catalogs
DROP TYPE typename
Inputs
typename The name of an existing type.
Outputs
DROP The message returned if the command is successful.
ERROR: RemoveType: type ’typename’ does not exist This message occurs if the specified
type is not found.

Description
DROP TYPE will remove a user type from the system catalogs.
Only the owner of a type can remove it.
Notes
DROP TYPE statement is a Postgres language extension.
Refer to CREATE TYPE for information on how to create types.
It is the user’s responsibility to remove any operators, functions, aggregates, access methods,
subtypes, and classes that use a deleted type.
If a built-in type is removed, the behavior of the backend is unpredictable.

Usage
To remove the box type:
DROP TYPE box;

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350

Compatibility
SQL3
DROP TYPE is a SQL3 statement.

D.44

DROP USER

Name
DROP USER â&#x20AC;&#x201D; Removes a user
DROP USER name
Inputs
name The name of an existing user.
Outputs
DROP USER The message returned if the user is successfully deleted.
ERROR: DROP USER: user "name" does not exist This message occurs if the username is
not found.
DROP USER: user "name " owns database "name", cannot be removed You must drop the
database first or change its ownership.

Description
DROP USER removes the specified user from the database. It does not remove tables, views, or
other objects owned by the user. If the user owns any database you get an error.
Use CREATE USER to add new users, and ALTER USER to change a userâ&#x20AC;&#x2122;s properties.
Postgres comes with a script dropuser which has the same functionality as this command (in fact,
it calls this command) but can be run from the command shell.

Usage
To drop a user account:
DROP USER jonathan;

D.45. DROP VIEW

351

Compatibility
SQL92
There is no DROP USER in SQL92.

D.45

DROP VIEW

Name
DROP VIEW â&#x20AC;&#x201D; Removes an existing view from a database
DROP VIEW name
Inputs
name The name of an existing view.
Outputs
DROP The message returned if the command is successful.
ERROR: RewriteGetRuleEventRel: rule "_RETname" not found This message occurs if the
specified view does not exist in the database.

Description
DROP VIEW drops an existing view from the database. To execute this command you must be
the owner of the view.
Notes
The Postgres DROP TABLE statement also drops views.
Refer to CREATE VIEW for information on how to create views.

Usage
This command will remove the view called kinds:
DROP VIEW kinds;

Inputs
RESTRICT Ensures that only a view with no dependent views or integrity constraints can be
destroyed.
CASCADE Any referencing views and integrity constraints will be dropped as well.
Notes
At present, to remove a referenced view from a Postgres database, you must drop it explicitly.

D.46

dropdb

Name
dropdb — Remove an existing Postgres database
dropdb [ options ] dbname
Inputs
-h, –host host Specifies the hostname of the machine on which the postmaster is running.
-p, –port port Specifies the Internet TCP/IP port or local Unix domain socket file extension on
which the postmaster is listening for connections.
-U, –username username Username to connect as.
-W, –password Force password prompt.
-e, –echo Echo the queries that dropdb generates and sends to the backend.
-q, –quiet Do not display a response.
-i, –interactive Issues a verification prompt before doing anything destructive.
dbname Specifies the name of the database to be removed. The database must be one of the
existing Postgres databases in this installation.
The options -h, -p, -U, -W, and -e are passed on literally to psql.

D.47. DROPLANG

353

Outputs
DROP DATABASE The database was successfully removed.
dropdb: Database removal failed. Something didn’t work out.
If there is an error condition, the backend error message will be displayed. See DROP DATABASE
and psql for possibilities.

Description
dropdb destroys an existing Postgres database. The user who executes this command must be a
database superuser or the owner of the database.
dropdb is a shell script wrapper around the SQL command DROP DATABASE via the Postgres
interactive terminal psql. Thus, there is nothing special about dropping databases via this or other
methods. This means that the psql must be found by the script and that a database server is
running at the targeted host. Also, any default settings and environment variables available to psql
and the libpq front-end library do apply.

droplang accepts the following command line arguments:
langname Specifies the name of the backend programming language to be removed. droplang
will prompt for langname if it is not specified on the command line.
[-d, –dbname dbname ] Specifies from which database the language should be removed.
-l, –list Shows a list of already installed languages in the target database (which must be specified).
droplang also accepts the following command line arguments for connection parameters:
-h, –host host Specifies the hostname of the machine on which the postmaster is running.
-p, –port port Specifies the Internet TCP/IP port or local Unix domain socket file extension on
which the postmaster is listening for connections.
-U, –username username Username to connect as.
-W, –password Force password prompt.
Outputs
Most error messages are self-explanatory. If not, run droplang with the –echo option and see under
the respective SQL command for details. Check also under psql for more possibilities.

Description
droplang is a utility for removing an existing programming language from a Postgres database.
droplang currently accepts two languages, plsql and pltcl.
Although backend programming languages can be removed directly using several SQL commands, it is recommended to use droplang because it performs a number of checks and is much
easier to use. See DROP LANGUAGE for more.

Notes
Use createlang to add a language.

Usage
To remove pltcl:
$ droplang pltcl

D.48. DROPUSER

D.48

355

dropuser

Name
dropuser — Drops (removes) a Postgres user
dropuser [ options ] [ username ]
Inputs
-h, –host host Specifies the hostname of the machine on which the postmaster is running.
-p, –port port Specifies the Internet TCP/IP port or local Unix domain socket file extension on
which the postmaster is listening for connections.
-e, –echo Echo the queries that createdb generates and sends to the backend.
-q, –quiet Do not display a response.
-i, –interactive Prompt for confirmation before actually removing the user.
username Specifies the name of the Postgres user to be removed. This name must exist in the
Postgres installation. You will be prompted for a name if none is specified on the command
line.
The options -h, -p, and -e, are passed on literally to psql. The psql options -U and -W are available
as well, but they can be confusing in this context.
Outputs
DROP USER All is well.
dropuser: deletion of user "username " failed Something went wrong. The user was not removed.
If there is an error condition, the backend error message will be displayed. See DROP USER and
psql for possibilities.

Description
dropuser removes an existing Postgres user and the databases which that user owned. Only users
with usesuper set in the pg_shadow class can destroy Postgres users.
dropuser is a shell script wrapper around the SQL command DROP USER via the Postgres
interactive terminal psql. Thus, there is nothing special about removing users via this or other
methods. This means that the psql must be found by the script and that a database server is
running at the targeted host. Also, any default settings and environment variables available to psql
and the libpq front-end library do apply.

To remove user joe using the postmaster on host eden, port 5000, with verification and a peek at
the underlying query:
$ dropuser -p 5000 -h eden -i -e joe
User "joe" and any owned databases will be permanently deleted. Are you sure?
(y/n) y
DROP USER "joe" DROP USER

D.49

ecpg

Name
ecpg â&#x20AC;&#x201D; Embedded SQL C preprocessor
ecpg [ -v ] [ -t ] [ -I include-path ] [ -o outfile ] file1 [ file2 ] [ ... ]
Inputs
ecpg accepts the following command line arguments:
-v Print version information.
-t Turn off auto-transaction mode.
-I path Specify an additional include path. Defaults are /usr/local/include, the Postgres include
path which is defined at compile time (default: /usr/local/pgsql/lib), and /usr/include.
-o Specifies that ecpg should write all its output to outfile. If no such option is given the output is
written to name.c, assuming the input file was named name.pgc. If the input file does have
the expected .pgc suffix, then the output file will have .pgc appended to the input file name.
file The files to be processed.
Outputs
ecpg will create a file or write to stdout.
return value ecpg returns 0 to the shell on successful completion, -1 for errors.

D.49. ECPG

357

Description
ecpg is an embedded SQL preprocessor for the C language and the Postgres. It enables development of C programs with embedded SQL code.
Linus Tolke was the original author of ecpg (up to version 0.2). Michael Meskes is the current
author and maintainer of ecpg. Thomas Good is the author of the last revision of the ecpg man
page, on which this document is based.

where the optional -d flag turns on debugging. The .pgc extension is an arbitrary means of denoting
ecpg source.
You may want to redirect the preprocessor output to a log file.
Compiling and Linking
Assuming the Postgres binaries are in /usr/local/pgsql, you will need to compile and link your
preprocessed source file:
gcc -g -I /usr/local/pgsql/include [ -o file ] file.c -L /usr/local/pgsql/lib -lecpg -lpq

Grammar
Libraries
The preprocessor will prepend two directives to the source:
#include <ecpgtype.h>
#include <ecpglib.h>

Note: Prior to version 2.1.0, each variable had to be declared on a separate line. As of version
2.1.0 multiple variables may be declared on a single line:
char foo(16), bar(16);

Error Handling
The SQL communication area is defined with:
EXEC SQL INCLUDE sqlca;

Note: The sqlca is in lowercase. While SQL convention may be followed, i.e., using uppercase to
separate embedded SQL from C statements, sqlca (which includes the sqlca.h header file) MUST
be lowercase. This is because the EXEC SQL prefix indicates that this INCLUDE will be parsed
by ecpg. ecpg observes case sensitivity (SQLCA.h will not be found). EXEC SQL INCLUDE
can be used to include other header files as long as case sensitivity is observed.
The sqlprint command is used with the EXEC SQL WHENEVER statement to turn on error
handling throughout the program:
EXEC SQL WHENEVER sqlerror sqlprint;

and
EXEC SQL WHENEVER not found sqlprint;

Note: This is not an exhaustive example of usage for the EXEC SQL WHENEVER statement.
Further examples of usage may be found in SQL manuals (e.g., The LAN TIMES Guide to SQL by
Groff and Weinberg).
Connecting to the Database Server
One connects to a database using the following:
EXEC SQL CONNECT dbname;

where the database name is not quoted. Prior to version 2.1.0, the database name was required to
be inside single quotes.
Specifying a server and port name in the connect statement is also possible. The syntax is:
dbname [@server ][:port ]

or
<tcp|unix> :postgresql://server [:port ][/dbname ][?options ]

D.49. ECPG

359

Queries
In general, SQL queries acceptable to other applications such as psql can be embedded into your C
code. Here are some examples of how to do that.
Create Table:
EXEC SQL CREATE TABLE foo (number int4, ascii char(16));
EXEC SQL CREATE UNIQUE index num1 on foo(number);
EXEC SQL COMMIT;

Notes
There is no EXEC SQL PREPARE statement.
The complete structure definition MUST be listed inside the declare section.
See the TODO file in the source for some more missing features.

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360

D.50

END

Name
END — Commits the current transaction
END [ WORK | TRANSACTION ]
Inputs
WORK, TRANSACTION Optional keywords. They have no effect.
Outputs
COMMIT Message returned if the transaction is successfully committed.
NOTICE: COMMIT: no transaction in progress If there is no transaction in progress.

Description
END is a Postgres extension, and is a synonym for the SQL92-compatible COMMIT.
Notes
The keywords WORK and TRANSACTION are noise and can be omitted.
Use ROLLBACK to abort a transaction.

Usage
To make all changes permanent:
END WORK;

Compatibility
SQL92
END is a PostgreSQL extension which provides functionality equivalent to COMMIT.

Description
This command displays the execution plan that the Postgres planner generates for the supplied
query. The execution plan shows how the table(s) referenced by the query will be scanned—by
plain sequential scan, index scan, etc.—and if multiple tables are referenced, what join algorithms
will be used to bring together the required tuples from each input table.
The most critical part of the display is the estimated query execution cost, which is the planner’s
guess at how long it will take to run the query (measured in units of disk page fetches). Actually
two numbers are shown: the start-up time before the first tuple can be returned, and the total time
to return all the tuples. For most queries the total time is what matters, but in contexts such as an
EXISTS sub-query the planner will choose the smallest start-up time instead of the smallest total
time (since the executor will stop after getting one tuple, anyway). Also, if you limit the number
of tuples to return with a LIMIT clause, the planner makes an appropriate interpolation between
the endpoint costs to estimate which plan is really the cheapest.
The VERBOSE option emits the full internal representation of the plan tree, rather than just
a summary (and sends it to the postmaster log file, too). Usually this option is only useful for
debugging Postgres.
Notes
There is only sparse documentation on the optimizer’s use of cost information in Postgres. General
information on cost estimation for query optimization can be found in database textbooks. Refer
to the Programmer’s Guide in the chapters on indexes and the genetic query optimizer for more
information.

Usage
To show a query plan for a simple query on a table with a single int4 column and 128 rows:
EXPLAIN SELECT * FROM foo;
NOTICE: QUERY PLAN:

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362

Seq Scan on foo (cost=0.00..2.28 rows=128 width=4)
EXPLAIN

For the same table with an index to support an equijoin condition on the query, EXPLAIN will
show a different plan:
EXPLAIN SELECT * FROM foo WHERE i = 4;
NOTICE: QUERY PLAN:
Index Scan using fi on foo (cost=0.00..0.42 rows=1 width=4)
EXPLAIN

And finally, for the same table with an index to support an equijoin condition on the query, EXPLAIN will show the following for a query using an aggregate function:
EXPLAIN SELECT sum(i) FROM foo WHERE i = 4;
NOTICE: QUERY PLAN:
Aggregate (cost=0.42..0.42 rows=1 width=4) > Index Scan using fi on foo (cost=0.00..0.42 rows=1 width=4)

Note that the specific numbers shown, and even the selected query strategy, may vary between
Postgres releases due to planner improvements.

RELATIVE Noise word for SQL92 compatibility.
count count determines how many rows to fetch. It can be one of the following:
# A signed integer that specifies how many rows to fetch. Note that a negative integer is
equivalent to changing the sense of FORWARD and BACKWARD.
ALL Retrieve all remaining rows.
NEXT Equivalent to specifying a count of 1.
PRIOR Equivalent to specifying a count of -1.
cursor An open cursorâ&#x20AC;&#x2122;s name.
Outputs
FETCH returns the results of the query defined by the specified cursor. The following messages
will be returned if the query fails:
NOTICE: PerformPortalFetch: portal "cursor " not found If cursor is not previously declared.
The cursor must be declared within a transaction block.
NOTICE: FETCH/ABSOLUTE not supported, using RELATIVE Postgres does not support
absolute positioning of cursors.
ERROR: FETCH/RELATIVE at current position is not supported SQL92 allows one to repetitively retrieve the cursor at its "current position" using the syntax FETCH RELATIVE 0
FROM cursor.
Postgres does not currently support this notion; in fact the value zero is reserved to indicate
that all rows should be retrieved and is equivalent to specifying the ALL keyword. If
the RELATIVE keyword has been used, Postgres assumes that the user intended SQL92
behavior and returns this error message.

Description
FETCH allows a user to retrieve rows using a cursor. The number of rows retrieved is specified
by #. If the number of rows remaining in the cursor is less than #, then only those available are
fetched. Substituting the keyword ALL in place of a number will cause all remaining rows in the
cursor to be retrieved. Instances may be fetched in both FORWARD and BACKWARD directions.
The default direction is FORWARD. Tip: Negative numbers are allowed to be specified for the row
count. A negative number is equivalent to reversing the sense of the FORWARD and BACKWARD
keywords. For example, FORWARD -1 is the same as BACKWARD 1.

APPENDIX D. REFERENCE MANUAL

364
Notes

Note that the FORWARD and BACKWARD keywords are Postgres extensions. The SQL92
syntax is also supported, specified in the second form of the command. See below for details on
compatibility issues.
Updating data in a cursor is not supported by Postgres, because mapping cursor updates back
to base tables is not generally possible, as is also the case with VIEW updates. Consequently,
users must issue explicit UPDATE commands to replace data.
Cursors may only be used inside of transactions because the data that they store spans multiple
user queries.
Use MOVE to change cursor position. DECLARE will define a cursor. Refer to BEGIN,
COMMIT, and ROLLBACK for further information about transactions.

Compatibility
SQL92
Note: The non-embedded use of cursors is a Postgres extension. The syntax and usage of cursors
is being compared against the embedded form of cursors defined in SQL92.
SQL92 allows absolute positioning of the cursor for FETCH, and allows placing the results into
explicit variables:
FETCH ABSOLUTE #
FROM cursor
INTO :variable [, ...]
ABSOLUTE The cursor should be positioned to the specified absolute row number. All row
numbers in Postgres are relative numbers so this capability is not supported.
:variable Target host variable(s).

D.53

GRANT

Name
GRANT — Grants access privilege to a user, a group or all users
GRANT privilege [, ...] ON object [, ...] TO { PUBLIC | GROUP group | username }
Inputs
privilege The possible privileges are:
SELECT Access all of the columns of a specific table/view.
INSERT Insert data into all columns of a specific table.
UPDATE Update all columns of a specific table.
DELETE Delete rows from a specific table.
RULE Define rules on the table/view (See CREATE RULE statement).
ALL Grant all privileges.
object The name of an object to which to grant access. The possible objects are:
• table
• view
• sequence
PUBLIC A short form representing all users.

APPENDIX D. REFERENCE MANUAL

366
GROUP group A group to whom to grant privileges.

username The name of a user to whom to grant privileges. PUBLIC is a short form representing
all users.
Outputs
CHANGE Message returned if successful.
ERROR: ChangeAcl: class "object " not found Message returned if the specified object is not
available or if it is impossible to give privileges to the specified group or users.

Description
GRANT allows the creator of an object to give specific permissions to all users (PUBLIC) or to a
certain user or group. Users other than the creator donâ&#x20AC;&#x2122;t have any access permission unless the
creator GRANTs permissions, after the object is created.
Once a user has a privilege on an object, he is enabled to exercise that privilege. There is
no need to GRANT privileges to the creator of an object, the creator automatically holds ALL
privileges, and can also drop the object.
Notes
Currently, to grant privileges in Postgres to only a few columns, you must create a view having
desired columns and then grant privileges to that view.
Use psql \z for further information about permissions on existing objects:
Database
= lusitania
+------------------+---------------------------------------------+
| Relation
|
Grant/Revoke Permissions
|
+------------------+---------------------------------------------+
| mytable
| {"=rw","miriam=arwR","group todos=rw"}
|
+------------------+---------------------------------------------+
Legend:
uname=arwR -- privileges granted to a user
group gname=arwR -- privileges granted to a GROUP
=arwR -- privileges granted to PUBLIC
r -- SELECT
w -- UPDATE/DELETE
a -- INSERT
R -- RULE
arwR -- ALL

D.53. GRANT

367

Refer to REVOKE statements to revoke access privileges.

Usage
Grant insert privilege to all users on table films:
GRANT INSERT ON films TO PUBLIC;

Grant all privileges to user manuel on view kinds:
GRANT ALL ON kinds TO manuel;

Fields are compatible with those in the Postgres implementation, with the following additions:
privilege SQL92 permits additional privileges to be specified:
SELECT
REFERENCES Allowed to reference some or all of the columns of a specific table/view in
integrity constraints.
USAGE Allowed to use a domain, character set, collation or translation. If an object specifies
anything other than a table/view, privilege must specify only USAGE.
object
[ TABLE table ] SQL92 allows the additional non-functional keyword TABLE.
CHARACTER SET Allowed to use the specified character set.
COLLATION Allowed to use the specified collation sequence.
TRANSLATION Allowed to use the specified character set translation.
DOMAIN Allowed to use the specified domain.
WITH GRANT OPTION Allowed to grant the same privilege to others.

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D.54

initdb

Name
initdb — Create a new Postgres database installation
initdb
[ –pgdata|-D dbdir ]
[ –sysid|-i sysid ]
[ –pwprompt|-W ]
[ –encoding|-E encoding ]
[ –pglib|-L libdir ]
[ –noclean | -n ]
[ –debug | -d ]
[ –template | -t ]
Inputs
–pgdata=dbdir , -D dbdir , PGDATA This option specifies where in the file system the database
should be stored. This is the only information required by initdb, but you can avoid it by setting the PGDATA environment variable, which can be convenient since the database server
(postmaster) can find the database directory later by the same variable.
–sysid=sysid , -i sysid Selects the system id of the database superuser. This defaults to the
effective user id of the user running initdb. It is really not important what the superuser’s
sysid is, but one might choose to start the numbering at some number like 0 or 1.
–pwprompt, -W Makes initdb prompt for a password of the database superuser. If you don’t plan
on using password authentication, this is not important. Otherwise you won’t be able to use
password authentication until you have a password set up.
–encoding=encoding , -E encoding Selects the multibyte encoding of the template database.
This will also be the default encoding of any database you create later, unless you override
it there. To use the multibyte encoding feature, you must specify so at build time, at which
time you also select the default for this option.
Other, less commonly used, parameters are also available:
–pglib=libdir , -l libdir initdb needs a few input files to initialize the database. This option
tells where to find them. You normally don’t have to worry about this since initdb knows
about the most common installation layouts and will find the files itself. You will be told
if you need to specify their location explicitly. If that happens, one of the files is called
global1.bki.source and is traditionally installed along with the others in the library directory
(e.g., /usr/local/pgsql/lib).

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369

–template, -t Replace the template1 database in an existing database system, and don’t touch
anything else. This is useful when you need to upgrade your template1 database using initdb
from a newer release of Postgres, or when your template1 database has become corrupted by
some system problem. Normally the contents of template1 remain constant throughout the
life of the database system. You can’t destroy anything by running initdb with the –template
option.
–noclean, -n By default, when initdb determines that an error prevented it from completely
creating the database system, it removes any files it may have created before determining
that it can’t finish the job. This option inhibits any tidying-up and is thus useful for debugging.
–debug, -d Print debugging output from the bootstrap backend and a few other messages of lesser
interest for the general public. The bootstrap backend is the program initdb uses to create
the catalog tables. This option generates a tremendous amount of output.
Outputs
initdb will create files in the specified data area which are the system tables and framework for a
complete installation.

Description
initdb creates a new Postgres database system. A database system is a collection of databases that
are all administered by the same Unix user and managed by a single postmaster.
Creating a database system consists of creating the directories in which the database data will
live, generating the shared catalog tables (tables that don’t belong to any particular database), and
creating the template1 database. When you create a new database, everything in the template1
database is copied. It contains catalog tables filled in for things like the built-in types.
You must not execute initdb as root. This is because you cannot run the database server as
root either, but the server needs to have access to the files initdb creates. Furthermore, during
the initialization phase, when there are no users and no access controls installed, Postgres will
only connect with the name of the current Unix user, so you must log in under the account that
will own the server process.
Although initdb will attempt to create the respective data directory, chances are that it won’t
have the permission to do so. Thus it is a good idea to create the data directory before running
initdb and to hand over the ownership of it to the database superuser.

directory Where in your Unix filesystem do you want alternate databases to go?
Outputs
initlocation will create directories in the specified place.

Description
initlocation creates a new Postgres secondary database storage area. See the discussion under
CREATE DATABASE about how to manage and use secondary storage areas. If the argument
does not contain a slash and is not valid as a path, it is assumed to be an environment variable,
which is referenced. See the examples at the end.
In order to use this command you must be logged in (using ’su’, for example) as the database
superuser.

Usage
To create a database in an alternate location, using an environment variable:
$ export PGDATA2=/opt/postgres/data

Start and stop postmaster so it sees the $PGDATA2 environment variable. The system must be
configured so the postmaster sees $PGDATA2 every time it starts.
$ initlocation PGDATA2
$ createdb -D ’PGDATA2’ ’testdb’

Inputs
table The name of an existing table.
column The name of a column in table.
expression A valid expression or value to assign to column.
query A valid query. Refer to the SELECT statement for a further description of valid arguments.
Outputs
INSERT oid 1 Message returned if only one row was inserted. oid is the numeric OID of the
inserted row.
INSERT 0 # Message returned if more than one rows were inserted. # is the number of rows
inserted.

Description
INSERT allows one to insert new rows into a class or table. One can insert a single row at a time
or several rows as a result of a query. The columns in the target list may be listed in any order.
Each column not present in the target list will be inserted using a default value, either a declared
DEFAULT value or NULL. Postgres will reject the new column if a NULL is inserted into a column
declared NOT NULL.
If the expression for each column is not of the correct data type, automatic type coercion will
be attempted.
You must have insert privilege to a table in order to append to it, as well as select privilege on
any table specified in a WHERE clause.

In this second example the column date_prod is omitted and therefore it will have the default value
of NULL:
INSERT INTO films (code, title, did, date_prod, kind)
VALUES (’T_601’, ’Yojimbo’, 106, DATE ’1961-06-16’, ’Drama’);

Insert a single row into table distributors; note that only column name is specified, so the omitted
column did will be assigned its default value:

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INSERT INTO distributors (name)
VALUES (’British Lion’);

Insert several rows into table films from table tmp:
INSERT INTO films
SELECT * FROM tmp;

Description
ipcclean cleans up shared memory and semaphore space from aborted backends by deleting all
instances owned by user postgres. Only the DBA should execute this program as it can cause
bizarre behavior (i.e., crashes) if run during multi-user execution. This program should be executed
if messages such as semget: No space left on device are encountered when starting up the postmaster
or the backend server.
If this command is executed while postmaster is running, the shared memory and semaphores
allocated by the postmaster will be deleted. This will result in a general failure of the backend
servers started by that postmaster.
This script is a hack, but in the many years since it was written, no one has come up with an
equally effective and portable solution. Suggestions are welcome.
The script makes assumption about the format of output of the ipcs utility which may not be
true across different operating systems. Therefore, it may not work on your particular OS.

D.58

LISTEN

Name
LISTEN â&#x20AC;&#x201D; Listen for a response on a notify condition
LISTEN name
Inputs
name Name of notify condition.
Outputs
LISTEN Message returned upon successful completion of registration.
NOTICE Async_Listen: We are already listening on name If this backend is already registered for that notify condition.

Description
LISTEN registers the current Postgres backend as a listener on the notify condition name.
Whenever the command NOTIFY name is invoked, either by this backend or another one
connected to the same database, all the backends currently listening on that notify condition are
notified, and each will in turn notify its connected frontend application. See the discussion of
NOTIFY for more information.
A backend can be unregistered for a given notify condition with the UNLISTEN command.
Also, a backendâ&#x20AC;&#x2122;s listen registrations are automatically cleared when the backend process exits.

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The method a frontend application must use to detect notify events depends on which Postgres
application programming interface it uses. With the basic libpq library, the application issues
LISTEN as an ordinary SQL command, and then must periodically call the routine PQnotifies to
find out whether any notify events have been received. Other interfaces such as libpgtcl provide
higher-level methods for handling notify events; indeed, with libpgtcl the application programmer
should not even issue LISTEN or UNLISTEN directly. See the documentation for the library
you are using for more details.
NOTIFY contains a more extensive discussion of the use of LISTEN and NOTIFY.
Notes
name can be any string valid as a name; it need not correspond to the name of any actual table. If
notifyname is enclosed in double-quotes, it need not even be a syntactically valid name, but can be
any string up to 31 characters long.
In some previous releases of Postgres, name had to be enclosed in double-quotes when it did
not correspond to any existing table name, even if syntactically valid as a name. That is no longer
required.

Outputs
LOAD Message returned on successful completion.
ERROR: LOAD: could not open file ’filename’ Message returned if the specified file is not
found. The file must be visible to the Postgres backend, with the appropriate full path name
specified, to avoid this message.

Description
Loads an object (or ".o") file into the Postgres backend address space. Once a file is loaded, all
functions in that file can be accessed. This function is used in support of user-defined types and
functions.
If a file is not loaded using LOAD, the file will be loaded automatically the first time the function
is called by Postgres. LOAD can also be used to reload an object file if it has been edited and
recompiled. Only objects created from C language files are supported at this time.
Notes
Functions in loaded object files should not call functions in other object files loaded through the
LOAD command. For example, all functions in file A should call each other, functions in the
standard or math libraries, or in Postgres itself. They should not call functions defined in a
different loaded file B. This is because if B is reloaded, the Postgres loader is not able to relocate
the calls from the functions in A into the new address space of B. If B is not reloaded, however,
there will not be a problem.
Object files must be compiled to contain position independent code. For example, on DECstations you must use /bin/cc with the -G 0 option when compiling object files to be loaded.
Note that if you are porting Postgres to a new platform, LOAD will have to work in order to
support ADTs.

ACCESS EXCLUSIVE MODE Note: Automatically acquired by ALTER TABLE, DROP TABLE, VACUUM statements. This is the most restrictive lock mode which conflicts with all
other lock modes and protects a locked table from any concurrent operations.
Note: This lock mode is also acquired by an unqualified LOCK TABLE (i.e., the command
without an explicit lock mode option).
Outputs
LOCK TABLE The lock was successfully applied.
ERROR name : Table does not exist. Message returned if name does not exist.

Description
LOCK TABLE controls concurrent access to a table for the duration of a transaction. Postgres
always uses the least restrictive lock mode whenever possible. LOCK TABLE provides for cases
when you might need more restrictive locking.
RDBMS locking uses the following terminology:
EXCLUSIVE Exclusive lock that prevents other locks from being granted.
SHARE Allows others to share lock. Prevents EXCLUSIVE locks.
ACCESS Locks table schema.
ROW Locks individual rows.
Note: If EXCLUSIVE or SHARE are not specified, EXCLUSIVE is assumed. Locks exist for the
duration of the transaction.
For example, an application runs a transaction at READ COMMITTED isolation level and needs
to ensure the existence of data in a table for the duration of the transaction. To achieve this you
could use SHARE lock mode over the table before querying. This will protect data from concurrent
changes and provide any further read operations over the table with data in their actual current
state, because SHARE lock mode conflicts with any ROW EXCLUSIVE one acquired by writers,
and your LOCK TABLE name IN SHARE MODE statement will wait until any concurrent
write operations commit or rollback. Note: To read data in their real current state when running a
transaction at the SERIALIZABLE isolation level you have to execute a LOCK TABLE statement
before executing any DML statement, when the transaction defines what concurrent changes will
be visible to itself.
In addition to the requirements above, if a transaction is going to change data in a table, then
SHARE ROW EXCLUSIVE lock mode should be acquired to prevent deadlock conditions when
two concurrent transactions attempt to lock the table in SHARE mode and then try to change

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data in this table, both (implicitly) acquiring ROW EXCLUSIVE lock mode that conflicts with a
concurrent SHARE lock.
To continue with the deadlock (when two transaction wait for one another) issue raised above,
you should follow two general rules to prevent deadlock conditions:
• Transactions have to acquire locks on the same objects in the same order.
For example, if one application updates row R1 and than updates row R2 (in the same
transaction) then the second application shouldn’t update row R2 if it’s going to update row
R1 later (in a single transaction). Instead, it should update rows R1 and R2 in the same order
as the first application.
• Transactions should acquire two conflicting lock modes only if one of them is self-conflicting
(i.e., may be held by one transaction at time only). If multiple lock modes are involved, then
transactions should always acquire the most restrictive mode first.
An example for this rule was given previously when discussing the use of SHARE ROW
EXCLUSIVE mode rather than SHARE mode.
Note: Postgres does detect deadlocks and will rollback at least one waiting transaction to resolve
the deadlock.
Notes
LOCK is a Postgres language extension.
Except for ACCESS SHARE/EXCLUSIVE lock modes, all other Postgres lock modes and the
LOCK TABLE syntax are compatible with those present in Oracle.
LOCK works only inside transactions.

Usage
Illustrate a SHARE lock on a primary key table when going to perform inserts into a foreign key
table:
BEGIN WORK;
LOCK TABLE films IN SHARE MODE;
SELECT id
FROM films
WHERE name = ’Star Wars: Episode I - The Phantom Menace’;
-- Do ROLLBACK if record was not returned
INSERT INTO films_user_comments
VALUES (_id_, ’GREAT! I was waiting for it for so long!’);
COMMIT WORK;

Description
MOVE allows a user to move cursor position a specified number of rows. MOVE works like the
FETCH command, but only positions the cursor and does not return rows.
Refer to FETCH for details on syntax and usage.
Notes
MOVE is a Postgres language extension.
Refer to FETCH for a description of valid arguments. Refer to DECLARE to define a cursor.
Refer to BEGIN, COMMIT, and ROLLBACK for further information about transactions.

Compatibility
SQL92
There is no SQL92 MOVE statement. Instead, SQL92 allows one to FETCH rows from an
absolute cursor position, implicitly moving the cursor to the correct position.

D.62

NOTIFY

Name
NOTIFY â&#x20AC;&#x201D; Signals all frontends and backends listening on a notify condition
NOTIFY name
Inputs
notifyname Notify condition to be signaled.
Outputs
NOTIFY Acknowledgement that notify command has executed.
Notify events Events are delivered to listening frontends; whether and how each frontend application reacts depends on its programming.

D.62. NOTIFY

381

Description
The NOTIFY command sends a notify event to each frontend application that has previously
executed LISTEN notifyname for the specified notify condition in the current database.
The information passed to the frontend for a notify event includes the notify condition name
and the notifying backend process’s PID. It is up to the database designer to define the condition
names that will be used in a given database and what each one means.
Commonly, the notify condition name is the same as the name of some table in the database,
and the notify event essentially means “I changed this table, take a look at it to see what’s new”.
But no such association is enforced by the NOTIFY and LISTEN commands. For example, a
database designer could use several different condition names to signal different sorts of changes
to a single table.
NOTIFY provides a simple form of signal or IPC (interprocess communication) mechanism for
a collection of processes accessing the same Postgres database. Higher-level mechanisms can be
built by using tables in the database to pass additional data (beyond a mere condition name) from
notifier to listener(s).
When NOTIFY is used to signal the occurrence of changes to a particular table, a useful
programming technique is to put the NOTIFY in a rule that is triggered by table updates. In this
way, notification happens automatically when the table is changed, and the application programmer
can’t accidentally forget to do it.
NOTIFY interacts with SQL transactions in some important ways. Firstly, if a NOTIFY is
executed inside a transaction, the notify events are not delivered until and unless the transaction is
committed. This is appropriate, since if the transaction is aborted we would like all the commands
within it to have had no effect, including NOTIFY. But it can be disconcerting if one is expecting
the notify events to be delivered immediately. Secondly, if a listening backend receives a notify
signal while it is within a transaction, the notify event will not be delivered to its connected frontend
until just after the transaction is completed (either committed or aborted). Again, the reasoning
is that if a notify were delivered within a transaction that was later aborted, one would want the
notification to be undone somehow—but the backend cannot "take back" a notify once it has sent
it to the frontend. So notify events are only delivered between transactions. The upshot of this is
that applications using NOTIFY for real-time signaling should try to keep their transactions short.
NOTIFY behaves like Unix signals in one important respect: if the same condition name is
signaled multiple times in quick succession, recipients may get only one notify event for several
executions of NOTIFY. So it is a bad idea to depend on the number of notifies received. Instead,
use NOTIFY to wake up applications that need to pay attention to something, and use a database
object (such as a sequence) to keep track of what happened or how many times it happened.
It is common for a frontend that sends NOTIFY to be listening on the same notify name itself.
In that case it will get back a notify event, just like all the other listening frontends. Depending on
the application logic, this could result in useless work —for example, re-reading a database table
to find the same updates that that frontend just wrote out. In Postgres 6.4 and later, it is possible
to avoid such extra work by noticing whether the notifying backend process’s PID (supplied in the

APPENDIX D. REFERENCE MANUAL

382

notify event message) is the same as one’s own backend’s PID (available from libpq). When they
are the same, the notify event is one’s own work bouncing back, and can be ignored. (Despite what
was said in the preceding paragraph, this is a safe technique. Postgres keeps self-notifies separate
from notifies arriving from other backends, so you cannot miss an outside notify by ignoring your
own notifies.)
Notes
name can be any string valid as a name; it need not correspond to the name of any actual table. If
name is enclosed in double-quotes, it need not even be a syntactically valid name, but can be any
string up to 31 characters long.
In some previous releases of Postgres, name had to be enclosed in double-quotes when it did
not correspond to any existing table name, even if syntactically valid as a name. That is no longer
required.
In Postgres releases prior to 6.4, the backend PID delivered in a notify message was always
the PID of the frontend’s own backend. So it was not possible to distinguish one’s own notifies
from other clients’ notifies in those earlier releases.

Inputs
-w Wait for the database server to come up, by watching for creation of the pid file (PGDATA/postmaster.pid). Times out after 60 seconds.
-D datadir Specifies the database location for this database installation.
-p path Specifies the path to the postmaster image.
-o "options " Specifies options to be passed directly to postmaster.
The parameters are usually surrounded by single or double quotes to ensure that they are
passed through as a group.
-m mode Specifies the shutdown mode.
smart, s smart mode waits for all the clients to logout. This is the default.
fast, f Fast mode sends SIGTERM to the backends; that means active transactions get rolled
back.
immediate, i Immediate mode sends SIGUSR1 to the backends and lets them abort. In this
case, database recovery will be necessary on the next start-up.
start Start up postmaster.
stop Shut down postmaster.
restart Restart the postmaster, performing a stop/start sequence.
status Show the current state of postmaster.
Outputs
pg_ctl: postmaster is state (pid: # ) Postmaster status.
If there is an error condition, the backend error message will be displayed.

Description
pg_ctl is a utility for starting, stopping or restarting postmaster.

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Usage
Starting postmaster
To start up postmaster:
$ pg_ctl start

If -w is supplied, pg_ctl waits for the database server to come up, by watching for creation of the
pid file (PGDATA/postmaster.pid), for up to 60 seconds.
Parameters to invoke postmaster are taken from the following sources:
• Path to postmaster: found in the command search path.
• Database directory: PGDATA environment variable.
• Other parameters: PGDATA/postmaster.opts.default.
postmaster.opts.default contains parameters for postmaster.
Note that postmaster.opts.default is installed by initdb from lib/postmaster.opts.default.sample under the Postgres installation directory (lib/postmaster.opts.default.sample is copied from src/bin/pg_ctl/postmaster.opts.default.sample while installing Postgres).
To override the default parameters you can use -D, -p and -o options.
An example of starting the postmaster, blocking until postmaster comes up is:
$ pg_ctl -w start

Stopping postmaster
$ pg_ctl stop stops postmaster. Using the -m switch allows one to control how the backend shuts
down. -w waits for postmaster to shut down. -m specifies the shut down mode.

D.64. PG_DUMP

385

Restarting postmaster
This is almost equivalent to stopping the postmaster then starting it again except that the parameters used before stopping it would be used too. This is done by saving them in $PGDATA/postmaster.opts file. -w, -D, -m, -fast, -immediate and -o can also be used in the restarting
mode and they have the same meanings as described above.
To restart postmaster in the simplest form:
$ pg_ctl restart

To restart postmaster, waiting for it to shut down and to come up:
$ pg_ctl -w restart

Inputs
pg_dump accepts the following command line arguments:
dbname Specifies the name of the database to be extracted. dbname defaults to the value of the
USER environment variable.
-a Dump out only the data, no schema (definitions).
-c Clean (drop) schema prior to create.
-d Dump data as proper insert strings.
-D Dump data as inserts with attribute names
-i Ignore version mismatch between pg_dump and the database server. Since pg_dump knows a
great deal about system catalogs, any given version of pg_dump is only intended to work with
the corresponding release of the database server. Use this option if you need to override the
version check (and if pg_dump then fails, donâ&#x20AC;&#x2122;t say you werenâ&#x20AC;&#x2122;t warned).
-n Suppress double quotes around identifiers unless absolutely necessary. This may cause trouble
loading this dumped data if there are reserved words used for identifiers. This was the default
behavior for pg_dump prior to v6.4.
-N Include double quotes around identifiers. This is the default.
-o Dump object identifiers (OIDs) for every table.
-s Dump out only the schema (definitions), no data.
-t table Dump data for table only.
-u Use password authentication. Prompts for username and password.
-v Specifies verbose mode.
-x Prevent dumping of ACLs (grant/revoke commands) and table ownership information.
pg_dump also accepts the following command line arguments for connection parameters:
-h host Specifies the hostname of the machine on which the postmaster is running. Defaults to
using a local Unix domain socket rather than an IP connection.
-p port Specifies the Internet TCP/IP port or local Unix domain socket file extension on which
the postmaster is listening for connections. The port number defaults to 5432, or the value
of the PGPORT environment variable (if set).
-u Use password authentication. Prompts for username and password.

D.64. PG_DUMP

387

Outputs
pg_dump will create a file or write to stdout.
Connection to database ’template1’ failed. connectDB() failed: Is the postmaster running
and accepting connections at ’UNIX Socket’ on port ’port’? pg_dump could not attach
to the postmaster process on the specified host and port. If you see this message, ensure
that the postmaster is running on the proper host and that you have specified the proper
port. If your site uses an authentication system, ensure that you have obtained the required
authentication credentials.
Connection to database ’dbname’ failed. FATAL 1: SetUserId: user ’username’ is not in
’pg_shadow’ You do not have a valid entry in the relation pg_shadow and and will not be
allowed to access Postgres. Contact your Postgres administrator.
dumpSequence(table ): SELECT failed You do not have permission to read the database. Contact your Postgres site administrator.
Note: pg_dump internally executes SELECT statements. If you have problems running pg_dump, make sure you are able to select information from the database using, for example, psql.

Description
pg_dump is a utility for dumping out a Postgres database into a script file containing query commands. The script files are in text format and can be used to reconstruct the database, even
on other machines and other architectures. pg_dump will produce the queries necessary to regenerate all user-defined types, functions, tables, indices, aggregates, and operators. In addition,
all the data is copied out in text format so that it can be readily copied in again, as well as imported
into tools for editing.
pg_dump is useful for dumping out the contents of a database to move from one Postgres
installation to another. After running pg_dump, one should examine the output script file for any
warnings, especially in light of the limitations listed below.

Notes
pg_dump has a few limitations. The limitations mostly stem from difficulty in extracting certain
meta-information from the system catalogs.
• pg_dump does not understand partial indices. The reason is the same as above; partial index
predicates are stored as plans.
• pg_dump does not handle large objects. Large objects are ignored and must be dealt with
manually.

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â&#x20AC;˘ When doing a data only dump, pg_dump emits queries to disable triggers on user tables
before inserting the data and queries to re-enable them after the data has been inserted. If
the restore is stopped in the middle, the system catalogs may be left in the wrong state.

-h host Specifies the hostname of the machine on which the postmaster is running. Defaults to
using a local Unix domain socket rather than an IP connection.
-p port Specifies the Internet TCP/IP port or local Unix domain socket file extension on which
the postmaster is listening for connections. The port number defaults to 5432, or the value
of the PGPORT environment variable (if set).
-u Use password authentication. Prompts for username and password.
Outputs
pg_dumpall will create a file or write to stdout.
Connection to database ’template1’ failed. connectDB() failed: Is the postmaster running
and accepting connections at ’UNIX Socket’ on port ’port ’? pg_dumpall could not attach to the postmaster process on the specified host and port. If you see this message,
ensure that the postmaster is running on the proper host and that you have specified the
proper port. If your site uses an authentication system, ensure that you have obtained the
required authentication credentials.
Connection to database ’dbname’ failed. FATAL 1: SetUserId: user ’username’ is not in
’pg_shadow’ You do not have a valid entry in the relation pg_shadow and and will not be
allowed to access Postgres. Contact your Postgres administrator.
dumpSequence(table ): SELECT failed You do not have permission to read the database. Contact your Postgres site administrator.
Note: pg_dumpall internally executes SELECT statements. If you have problems running pg_dumpall, make sure you are able to select information from the database using, for example,
psql.

Description
pg_dumpall is a utility for dumping out all Postgres databases into one file. It also dumps the
pg_shadow table, which is global to all databases. pg_dumpall includes in this file the proper
commands to automatically create each dumped database before loading.
pg_dumpall takes all pg_dump options, but -f, -t and dbname should be omitted.
Refer to pg_dump for more information on this capability.

Usage
To dump all databases:
$ pg_dumpall > db.out

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Tip: You can use most pg_dump options for pg_dumpall.
To reload this database:
$ psql -e template1 < db.out

Description
pg_passwd is a tool to manipulate the flat password file functionality of Postgres. This style of
password authentication is not required in an installation, but is one of several supported security
mechanisms.
Specify the password file in the same style of Ident authentication in $PGDATA/pg_hba.conf:
host unv 133.65.96.250 255.255.255.255 password passwd

where the above line allows access from 133.65.96.250 using the passwords listed in $PGDATA/passwd. The format of the password file follows those of /etc/passwd and /etc/shadow.
The first field is the user name, and the second field is the encrypted password. The rest is
completely ignored. Thus the following three sample lines specify the same user and password
pair:
pg_guest:/nB7.w5Auq.BY:10031::::::
pg_guest:/nB7.w5Auq.BY:93001:930::/home/guest:/bin/tcsh
pg_guest:/nB7.w5Auq.BY:93001

Supply the password file to the pg_passwd command. In the case described above, after changing
the working directory to PGDATA, the following command execution specifies the new password
for pg_guest:
$ pg_passwd passwd
Username: pg_guest
Password:
Re-enter password:

D.67. PG_UPGRADE

391

where the Password: and Re-enter password: prompts require the same password input which are
not displayed on the terminal. The original password file is renamed to passwd.bk.
psql uses the -u option to invoke this style of authentication.
The following lines show the sample usage of the option:
$ psql -h hyalos -u unv
Username: pg_guest
Password:
Welcome to the POSTGRESQL interactive sql monitor:
Please read the file COPYRIGHT for copyright terms of POSTGRESQL
type \? for help on slash commands
type \q to quit
type \g or terminate with semicolon to execute query
You are currently connected to the database: unv
unv=>

Description
pg_upgrade is a utility for upgrading from a previous Postgres release without reloading all the
data. Not all Postgres release transitions can be handled this way. Check the release notes for
details on your installation.

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Upgrading Postgres with pg_upgrade

1. Back up your existing data directory, preferably by making a complete dump with pg_dumpall.
2. Then do:
$ pg_dumpall -s > db.out

3. to dump out your old databaseâ&#x20AC;&#x2122;s table definitions without any data.
4. Stop the old postmaster and all backends.
5. Rename (using mv) your old pgsql data/ directory to data.old/.
6. Do:
$ make install

7. to install the new binaries.
8. Run initdb to create a new template1 database containing the system tables for the new
release.
9. Start the new postmaster. (Note: it is critical that no users connect to the database until the
upgrade is complete. You may wish to start the postmaster without -i and/or alter pg_hba.conf
temporarily.)
10. Change your working directory to the pgsql main directory, and type:
$ pg_upgrade -f db.out data.old

11. The program will do some checking to make sure everything is properly configured, and
will run your db.out script to recreate all the databases and tables you had, but with no data.
It will then physically move the data files containing non-system tables and indexes from
data.old/ into the proper data/ subdirectories, replacing the empty data files created during
the db.out script.
12. Restore your old pg_hba.conf if needed to allow user logins.
13. Stop and restart the postmaster.
14. Carefully examine the contents of the upgraded database. If you detect problems, youâ&#x20AC;&#x2122;ll need
to recover by restoring from your full pg_dump backup. You can delete the data.old/ directory
when you are satisfied.
15. The upgraded database will be in an un-vacuumed state. You will probably want to run a
VACUUM ANALYZE before beginning production work.

Description
pgtksh provides a graphical TCL/TK shell interface for Postgres.
Another way of accessing Postgres through TCL is to use pgtclsh or pgaccess.

D.71

postgres

Name
postgres â&#x20AC;&#x201D; Run a Postgres single-user backend
postgres [ dbname ]
postgres [ -B nBuffers ] [ -C ] [ -D DataDir ] [ -E ] [ -F ] [ -O ] [ -P ] [ -Q ] [ -S SortSize ] [ -d [
DebugLevel ] ] [ -e ] [ -o ] [ OutputFile ] [ -s ] [ -v protocol ] [ dbname ]
Inputs
postgres accepts the following command line arguments:
dbname The optional argument dbname specifies the name of the database to be accessed. dbname
defaults to the value of the USER environment variable.
-B nBuffers If the backend is running under the postmaster, nBuffers is the number of sharedmemory buffers that the postmaster has allocated for the backend server processes that it
starts. If the backend is running stand-alone, this specifies the number of buffers to allocate.
This value defaults to 64 buffers, where each buffer is 8k bytes (or whatever BLCKSZ is set
to in config.h).
-C Do not show the server version number.

D.71. POSTGRES

397

-D DataDir Specifies the directory to use as the root of the tree of database directories. If
-D is not given, the default data directory name is the value of the environment variable
PGDATA. If PGDATA is not set, then the directory used is $POSTGRESHOME/data. If
neither environment variable is set and this command-line option is not specified, the default
directory that was set at compile-time is used.
-E Echo all queries.
-F Disable an automatic fsync() call after each transaction. This option improves performance,
but an operating system crash while a transaction is in progress may cause the loss of the
most recently entered data. Without the fsync() call the data is buffered by the operating
system, and written to disk sometime later.
-O Override restrictions, so system table structures can be modified. These tables are typically
those with a leading pg_ in the table name.
-P Ignore system indexes to scan/update system tuples. The REINDEX for system tables/indexes
requires this option. System tables are typically those with a leading pg_ in the table name.
-Q Specifies "quiet" mode.
-S SortSize Specifies the amount of memory to be used by internal sorts and hashes before
resorting to temporary disk files. The value is specified in kilobytes, and defaults to 512
kilobytes. Note that for a complex query, several sorts and/or hashes might be running in
parallel, and each one will be allowed to use as much as SortSize kilobytes before it starts to
put data into temporary files.
-d [ DebugLevel ] The optional argument DebugLevel determines the amount of debugging output the backend servers will produce. If DebugLevel is one, the postmaster will trace all
connection traffic, and nothing else. For levels two and higher, debugging is turned on in
the backend process and the postmaster displays more information, including the backend
environment and process traffic. Note that if no file is specified for backend servers to send
their debugging output then this output will appear on the controlling tty of their parent
postmaster.
-e This option controls how dates are interpreted upon input to and output from the database.
If the -e option is supplied, then dates passed to and from the frontend processes will be
assumed to be in "European" format (DD-MM-YYYY), otherwise dates are assumed to be
in "American" format (MM-DD-YYYY). Dates are accepted by the backend in a wide variety
of formats, and for input dates this switch mostly affects the interpretation for ambiguous
cases. See the PostgreSQL Userâ&#x20AC;&#x2122;s Guide for more information.
-o OutputFile Sends all debugging and error output to OutputFile. If the backend is running
under the postmaster, error messages are still sent to the frontend process as well as to

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OutputFile, but debugging output is sent to the controlling tty of the postmaster (since only
one file descriptor can be sent to an actual file).
-s Print time information and other statistics at the end of each query. This is useful for benchmarking or for use in tuning the number of buffers.
-v protocol Specifies the number of the frontend/backend protocol to be used for this particular
session.
There are several other options that may be specified, used mainly for debugging purposes. These
are listed here only for the use by Postgres system developers. Use of any of these options is highly
discouraged. Furthermore, any of these options may disappear or change at any time.
These special-case options are:
-A [ n | r | b | Q | X ] This option generates a tremendous amount of output.
-L Turns off the locking system.
-N Disables use of newline as a query delimiter.
-f [ s | i | m | n | h ] Forbids the use of particular scan and join methods: s and i disable sequential and index scans respectively, while n, m, and h disable nested-loop, merge and hash
joins respectively. Note: Neither sequential scans nor nested-loop joins can be disabled
completely; the -fs and -fn options simply discourage the optimizer from using those plan
types if it has any other alternative.
-i Prevents query execution, but shows the plan tree.
-p dbname Indicates to the backend server that it has been started by a postmaster and makes
different assumptions about buffer pool management, file descriptors, etc. Switches following
-p are restricted to those considered "secure".
-t pa[rser | pl[anner] | e[xecutor]] Print timing statistics for each query relating to each of the
major system modules. This option cannot be used with -s.
Outputs
Of the nigh-infinite number of error messages you may see when you execute the backend server
directly, the most common will probably be:
semget: No space left on device If you see this message, you should run the ipcclean command.
After doing this, try starting postmaster again. If this still doesnâ&#x20AC;&#x2122;t work, you probably need
to configure your kernel for shared memory and semaphores as described in the installation
notes. If you have a kernel with particularly small shared memory and/or semaphore limits,
you may have to reconfigure your kernel to increase its shared memory or semaphore
parameters. Tip: You may be able to postpone reconfiguring your kernel by decreasing -B
to reduce Postgresâ&#x20AC;&#x2122; shared memory consumption.

D.72. POSTMASTER

399

Description
The Postgres backend server can be executed directly from the user shell. This should be done
only while debugging by the DBA, and should not be done while other Postgres backends are being
managed by a postmaster on this set of databases.
Some of the switches explained here can be passed to the backend through the "database
options" field of a connection request, and thus can be set for a particular backend without going to
the trouble of restarting the postmaster. This is particularly handy for debugging-related switches.
The optional argument dbname specifies the name of the database to be accessed. dbname
defaults to the value of the USER environment variable.

Notes
Useful utilities for dealing with shared memory problems include ipcs(1), ipcrm(1), and ipcclean(1).
See also postmaster.

D.72

postmaster

Name
postmaster â&#x20AC;&#x201D; Run the Postgres multi-user backend
postmaster [ -B nBuffers ] [ -D DataDir ] [ -N maxBackends ] [ -S ] [ -d DebugLevel ] [ -i ] [ -l ] [
-o BackendOptions ] [ -p port ] [ -n | -s ]
Inputs
postmaster accepts the following command line arguments:
-B nBuffers Sets the number of shared-memory disk buffers for the postmaster to allocate for
use by the backend server processes that it starts. This value defaults to 64 buffers, where
each buffer is 8k bytes (or whatever BLCKSZ is set to in src/include/config.h).
-D DataDir Specifies the directory to use as the root of the tree of database directories. If
-D is not given, the default data directory name is the value of the environment variable
PGDATA. If PGDATA is not set, then the directory used is $POSTGRESHOME/data. If
neither environment variable is set and this command-line option is not specified, the default
directory that was set at compile-time is used.
-N maxBackends Sets the maximum number of backend server processes that this postmaster
is allowed to start. By default, this value is 32, but it can be set as high as 1024 if your system
will support that many processes. (Note that -B is required to be at least twice -N, so youâ&#x20AC;&#x2122;ll
need to increase -B if you increase -N.) Both the default and upper limit values for -N can be
altered when building Postgres (see src/include/config.h).

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-S Specifies that the postmaster process should start up in silent mode. That is, it will disassociate
from the userâ&#x20AC;&#x2122;s (controlling) tty, start its own process group, and redirect its standard output
and standard error to /dev/null.
Note that using this switch makes it very difficult to troubleshoot problems, since all tracing
and logging output that would normally be generated by this postmaster and its child backends
will be discarded.
-d DebugLevel Determines the amount of debugging output the backend servers will produce.
If DebugLevel is one, the postmaster will trace all connection traffic. Levels two and higher
turn on increasing amounts of debug output from the backend processes, and the postmaster
displays more information including the backend environment and process traffic. Note that
unless the postmasterâ&#x20AC;&#x2122;s standard output and standard error are redirected into a log file, all
this output will appear on the controlling tty of the postmaster.
-i Allows clients to connect via TCP/IP (Internet domain) connections. Without this option, only
local Unix domain socket connections are accepted.
-l Enables secure connections using SSL. The -i option is also required. You must have compiled
with SSL enabled to use this option.
-o BackendOptions The postgres option(s) specified in BackendOptions are passed to all backend
server processes started by this postmaster. If the option string contains any spaces, the
entire string must be quoted.
-p port Specifies the TCP/IP port or local Unix domain socket file extension on which the postmaster is to listen for connections from frontend applications. Defaults to the value of the
PGPORT environment variable, or if PGPORT is not set, then defaults to the value established when Postgres was compiled (normally 5432). If you specify a port other than the
default port, then all frontend applications (including psql) must specify the same port using
either command-line options or PGPORT.
Two additional command line options are available for debugging problems that cause a backend to
die abnormally. These options control the behavior of the postmaster in this situation, and neither
option is intended for use in ordinary operation.
The ordinary strategy for this situation is to notify all other backends that they must terminate
and then reinitialize the shared memory and semaphores. This is because an errant backend could
have corrupted some shared state before terminating.
These special-case options are:
-n postmaster will not reinitialize shared data structures. A knowledgeable system programmer
can then use a debugger to examine shared memory and semaphore state.

D.72. POSTMASTER

401

-s postmaster will stop all other backend processes by sending the signal SIGSTOP, but will not
cause them to terminate. This permits system programmers to collect core dumps from all
backend processes by hand.
Outputs
semget: No space left on device If you see this message, you should run the ipcclean command.
After doing so, try starting postmaster again. If this still doesnâ&#x20AC;&#x2122;t work, you probably need
to configure your kernel for shared memory and semaphores as described in the installation
notes. If you run multiple instances of postmaster on a single host, or have a kernel with
particularly small shared memory and/or semaphore limits, you may have to reconfigure
your kernel to increase its shared memory or semaphore parameters. Tip: You may be able
to postpone reconfiguring your kernel by decreasing -B to reduce Postgresâ&#x20AC;&#x2122; shared memory
consumption, and/or by reducing -N to reduce Postgresâ&#x20AC;&#x2122; semaphore consumption.
StreamServerPort: cannot bind to port If you see this message, you should make certain that
there is no other postmaster process already running on the same port number. The easiest
way to determine this is by using the command $ ps -ax | grep postmaster on BSD-based
systems, or $ ps -e | grep postmast for System V-like or POSIX-compliant systems such
as HP-UX.
If you are sure that no other postmaster processes are running and you still get this error, try
specifying a different port using the -p option. You may also get this error if you terminate
the postmaster and immediately restart it using the same port; in this case, you must simply
wait a few seconds until the operating system closes the port before trying again. Finally,
you may get this error if you specify a port number that your operating system considers to
be reserved. For example, many versions of Unix consider port numbers under 1024 to be
trusted and only permit the Unix superuser to access them.
IpcMemoryAttach: shmat() failed: Permission denied A likely explanation is that another
user attempted to start a postmaster process on the same port which acquired shared
resources and then died. Since Postgres shared memory keys are based on the port number
assigned to the postmaster, such conflicts are likely if there is more than one installation on
a single host. If there are no other postmaster processes currently running (see above), run
ipcclean and try again. If other postmaster images are running, you will have to find the
owners of those processes to coordinate the assignment of port numbers and/or removal of
unused shared memory segments.

Description
postmaster manages the communication between frontend and backend processes, as well as
allocating the shared buffer pool and SysV semaphores (on machines without a test-and-set in-

APPENDIX D. REFERENCE MANUAL

402

struction). postmaster does not itself interact with the user and should be started as a background
process.
Only one postmaster should be running at a time in a given Postgres installation. Here, an
installation means a database directory and postmaster port number. You can run more than one
postmaster on a machine only if each one has a separate directory and port number.

Notes
If at all possible, do not use SIGKILL when killing the postmaster. SIGHUP, SIGINT, or SIGTERM
(the default signal for kill(1))" should be used instead. Using $ kill -KILL or its alternative form
$ kill -9 will prevent postmaster from freeing the system resources (e.g., shared memory and
semaphores) that it holds before dying. Use SIGTERM instead to avoid having to clean up manually
(as described earlier).
Useful utilities for dealing with shared memory problems include ipcs(1), ipcrm(1), and ipcclean(1).

This command will start up postmaster on the default port (5432). This is the simplest and most
common way to start the postmaster.
To start postmaster with a specific port:
$ nohup postmaster -p 1234 &

This command will start up postmaster communicating through the port 1234. In order to connect
to this postmaster using psql, you would need to run it as:
$ psql -p 1234

Summary
psql is a terminal-based front-end to Postgres. It enables you to type in queries interactively, issue
them to Postgres, and see the query results. Alternatively, input can be from a file. In addition, it
provides a number of meta-commands and various shell-like features to facilitate writing scripts
and automating a wide variety of tasks.

Description
Connecting To A Database
psql is a regular Postgres client application. In order to connect to a database you need to know
the name of your target database, the hostname and port number of the server and what user
name you want to connect as. psql can be told about those parameters via command line options,
namely -d, -h, -p, and -U respectively. If an argument is found that does not belong to any option
it will be interpreted as the database name (or the user name, if the database name is also given).
Not all these options are required, defaults do apply. If you omit the host name psql will connect
via a Unix domain socket to a server on the local host. The default port number is compile-time
determined. Since the database server uses the same default, you will not have to specify the port
in most cases. The default user name is your Unix username, as is the default database name.
Note that you can’t just connect to any database under any username. Your database administrator
should have informed you about your access rights. To save you some typing you can also set the
environment variables PGDATABASE, PGHOST, PGPORT and PGUSER to appropriate values.
If the connection could not be made for any reason (e.g., insufficient privileges, postmaster is
not running on the server, etc.), psql will return an error and terminate.
Entering Queries
In normal operation, psql provides a prompt with the name of the database to which psql is currently
connected, followed by the string “=>”. For example,
$ psql testdb
Welcome to psql, the PostgreSQL interactive terminal.
Type: \copyright for distribution terms
\h for help with SQL commands
\? for help on internal slash commands
\g or terminate with semicolon to execute query
\q to quit
testdb=>

At the prompt, the user may type in SQL queries. Ordinarily, input lines are sent to the backend
when a query-terminating semicolon is reached. An end of line does not terminate a query! Thus

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APPENDIX D. REFERENCE MANUAL

queries can be spread over several lines for clarity. If the query was sent and without error, the
query results are displayed on the screen.
Whenever a query is executed, psql also polls for asynchronous notification events generated
by LISTEN and NOTIFY.

psql Meta-Commands
Anything you enter in psql that begins with an unquoted backslash is a psql meta-command that is
processed by psql itself. These commands are what makes psql interesting for administration or
scripting. Meta-commands are more commonly called slash or backslash commands.
The format of a psql command is the backslash, followed immediately by a command verb,
then any arguments. The arguments are separated from the command verb and each other by any
number of whitespace characters.
To include whitespace into an argument you must quote it with a single quote. To include a
single quote into such an argument, precede it by a backslash. Anything contained in single quotes
is furthermore subject to C-like substitutions for \n (new line), \t (tab), \digits, \0digits, and \0xdigits
(the character with the given decimal, octal, or hexadecimal code).
If an unquoted argument begins with a colon (:), it is taken as a variable and the value of the
variable is taken as the argument instead.
Arguments that are quoted in backticks (`) are taken as a command line that is passed to the
shell. The output of the command (with a trailing newline removed) is taken as the argument
value. The above escape sequences also apply in backticks.
Some commands take the name of an SQL identifier (such as a table name) as argument. These
arguments follow the syntax rules of SQL regarding double quotes: an identifier without double
quotes is coerced to lower-case. For all other commands double quotes are not special and will
become part of the argument.
Parsing for arguments stops when another unquoted backslash occurs. This is taken as the
beginning of a new meta-command. The special sequence \ \ (two backslashes) marks the end of
arguments and continues parsing SQL queries, if any. That way SQL and psql commands can be
freely mixed on a line. But in any case, the arguments of a meta-command cannot continue beyond
the end of the line.
The following meta-commands are defined:
\a If the current table output format is unaligned, switch to aligned. If it is not unaligned, set it
to unaligned. This command is kept for backwards compatibility. See \pset for a general
solution.
\C [ title ] Set the title of any tables being printed as the result of a query or unset any such title.
This command is equivalent to \pset title title. (The name of this command derives from
caption, as it was previously only used to set the caption in an HTML table.)

D.73. PSQL

405

\connect (or \c) [ dbname [ username ]] Establishes a connection to a new database and/or
under a user name. The previous connection is closed. If dbname is - the current database
name is assumed.
If username is omitted the current user name is assumed.
As a special rule, \connect without any arguments will connect to the default database as
the default user (as you would have gotten by starting psql without any arguments).
If the connection attempt failed (wrong username, access denied, etc.), the previous connection will be kept if and only if psql is in interactive mode. When executing a non-interactive
script, processing will immediately stop with an error. This distinction was chosen as a user
convenience against typos on the one hand, and a safety mechanism that scripts are not
accidentally acting on the wrong database on the other hand.
\copy table [ with oids { from | to } filename | stdin | stdout [ using delimiters ’characters’
] [ with null as ’string’ ]] Performs a frontend (client) copy. This is an operation that runs
an SQL COPY command, but instead of the backend’s reading or writing the specified file,
and consequently requiring backend access and special user privilege, as well as being bound
to the file system accessible by the backend, psql reads or writes the file and routes the data
between the backend and the local file system.
The syntax of the command is similar to that of the SQL COPY command (see its description
for the details). Note that, because of this, special parsing rules apply to the \copy command.
In particular, the variable substitution rules and backslash escapes do not apply.
Tip: This operation is not as efficient as the SQL COPY command because all data must
pass through the client/server IP or socket connection. For large amounts of data the other
technique may be preferable.
Note: Note the difference in interpretation of stdin and stdout between frontend and backend
copies: in a frontend copy these always refer to psql’s input and output stream. On a backend
copy stdin comes from wherever the COPY itself came from (for example, a script run with
the -f option), and stdout refers to the query output stream (see \o meta-command below).
\copyright Shows the copyright and distribution terms of Postgres.
\d relation Shows all columns of relation (which could be a table, view, index, or sequence), their
types, and any special attributes such as NOT NULL or defaults, if any. If the relation is, in
fact, a table, any defined indices are also listed. If the relation is a view, the view definition
is also shown.
The command form \d+ is identical, but any comments associated with the table columns
are shown as well.
Note: If \d is called without any arguments, it is equivalent to \dtvs which will show a list
of all tables, views, and sequences. This is purely a convenience measure.

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\da [ pattern ] Lists all available aggregate functions, together with the data type they operate
on. If pattern (a regular expression) is specified, only matching aggregates are shown.
\dd [ object ] Shows the descriptions of object (which can be a regular expression), or of all objects
if no argument is given. (Object covers aggregates, functions, operators, types, relations
(tables, views, indices, sequences, large objects), rules, and triggers.) For example:
=> \dd version
Object descriptions
Name | What
| Description
---------+----------+--------------------------version | function | PostgreSQL version string
(1 row)

Descriptions for objects can be generated with the COMMENT ON SQL command.
Note: Postgres stores the object descriptions in the pg_description system table.
\df [ pattern ] Lists available functions, together with their argument and return types. If pattern
(a regular expression) is specified, only matching functions are shown. If the form \df+
is used, additional information about each function, including language and description, is
shown.
\distvS [ pattern ] This is not the actual command name: The letters i, s, t, v, S stand for index,
sequence, table, view, and system table, respectively. You can specify any or all of them in
any order to obtain a listing of them, together with who the owner is.
If pattern is specified, it is a regular expression that restricts the listing to those objects
whose name matches. If one appends a + to the command name, each object is listed with
its associated description, if any.
\dl This is an alias for \lo_list, which shows a list of large objects.
\do [ name ] Lists available operators with their operand and return types. If name is specified,
only operators with that name will be shown.
\dp [ pattern ] This is an alias for \z which was included for its greater mnemonic value (display
permissions).
\dT [ pattern ] Lists all data types or only those that match pattern. The command form \dT+
shows extra information.
\edit (or \e) [ filename ] If filename is specified, the file is edited; after the editor exits, its content
is copied back to the query buffer. If no argument is given, the current query buffer is copied
to a temporary file which is then edited in the same fashion.

D.73. PSQL

407

The new query buffer is then re-parsed according to the normal rules of psql, where the
whole buffer is treated as a single line. (Thus you cannot make scripts this way. Use \i for
that.) This means also that if the query ends with (or rather contains) a semicolon, it is
immediately executed. In other cases it will merely wait in the query buffer.
Tip: psql searches the environment variables PSQL_EDITOR, EDITOR, and VISUAL (in
that order) for an editor to use. If all of them are unset, /bin/vi is run.
\echo text [ ... ] Prints the arguments to the standard output, separated by one space and followed
by a newline. This can be useful to intersperse information in the output of scripts. For
example:
=> \echo â&#x20AC;&#x2DC;dateâ&#x20AC;&#x2DC;
Tue Oct 26 21:40:57 CEST 1999

If the first argument is an unquoted -n the the trailing newline is not written.
Tip: If you use the \o command to redirect your query output you may wish to use \qecho
instead of this command.
\encoding [ encoding ] Sets the client encoding, if you are using multibyte encodings. Without
an argument, this command shows the current encoding.
\f [ string ] Sets the field separator for unaligned query output. The default is pipe ( | ). See also
\pset for a generic way of setting output options.
\g [ {filename | |command} ] Sends the current query input buffer to the backend and optionally
saves the output in filename or pipes the output into a separate Unix shell to execute command.
A bare \g is virtually equivalent to a semicolon. A \g with argument is a one-shot alternative
to the \o command.
\help (or \h) [ command ] Give syntax help on the specified SQL command. If command is not
specified, then psql will list all the commands for which syntax help is available. If command
is an asterisk (*), then syntax help on all SQL commands is shown.
Note: To simplify typing, commands that consists of several words do not have to be quoted.
Thus it is fine to type \help alter table.
\H Turns on HTML query output format. If the HTML format is already on, it is switched back
to the default aligned text format. This command is for compatibility and convenience, but
see \pset about setting other output options.
\i filename Reads input from the file filename and executes it as though it had been typed on the
keyboard.
Note: If you want to see the lines on the screen as they are read you must set the variable
ECHO to all.

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\l (or \list) List all the databases in the server as well as their owners. Append a + to the command
name to see any descriptions for the databases as well. If your Postgres installation was
compiled with multibyte encoding support, the encoding scheme of each database is shown
as well.
\lo_export loid filename Reads the large object with OID loid from the database and writes it to
filename. Note that this is subtly different from the server function lo_export, which acts
with the permissions of the user that the database server runs as and on the server’s file
system.
Tip: Use \lo_list to find out the large object’s OID.
Note: See the description of the LO_TRANSACTION variable for important information
concerning all large object operations.
\lo_import filename [ comment ] Stores the file into a Postgres large object. Optionally, it associates the given comment with the object. Example:
foo=> \lo_import ’/home/peter/pictures/photo.xcf’ ’a picture of me’
lo_import 152801

The response indicates that the large object received object id 152801 which one ought to remember
if one wants to access the object ever again. For that reason it is recommended to always associate
a human-readable comment with every object. Those can then be seen with the \lo_list command.
Note that this command is subtly different from the server-side lo_import because it acts as
the local user on the local file system, rather than the server’s user and file system.
Note: See the description of the LO_TRANSACTION variable for important information
concerning all large object operations.
\lo_list Shows a list of all Postgres large objects currently stored in the database along with their
owners.
\lo_unlink loid Deletes the large object with OID loid from the database.
Tip: Use \lo_list to find out the large object’s OID.
Note: See the description of the LO_TRANSACTION variable for important information
concerning all large object operations.
\o [ {filename | |command} ] Saves future query results to the file filename or pipes future
results into a separate Unix shell to execute command. If no arguments are specified, the
query output will be reset to stdout.
Query results includes all tables, command responses, and notices obtained from the database
server, as well as output of various backslash commands that query the database (such as \d
), but not error messages.
Tip: To intersperse text output in between query results, use \qecho.

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\p Print the current query buffer to the standard output.
\pset parameter [ value ] This command sets options affecting the output of query result tables.
parameter describes which option is to be set. The semantics of value depend thereon.
Adjustable printing options are:
format Sets the output format to one of unaligned, aligned, html, or latex. Unique abbreviations
are allowed. (That would mean one letter is enough.)
Unaligned writes all fields of a tuple on a line, separated by the currently active field separator.
This is intended to create output that might be intended to be read in by other programs
(tab-separated, comma-separated).
Aligned mode is the standard, human-readable, nicely formatted text output that is default.
The HTML and LATEX modes put out tables that are intended to be included in documents
using the respective mark-up language. They are not complete documents! (This might not
be so dramatic in HTML, but in LATEX you must have a complete document wrapper.)
border The second argument must be a number. In general, the higher the number the more
borders and lines the tables will have, but this depends on the particular format. In HTML
mode, this will translate directly into the border=... attribute, in the others only values 0
(no border), 1 (internal dividing lines), and 2 (table frame) make sense.
expanded (or x) Toggles between regular and expanded format. When expanded format is enabled, all output has two columns with the field name on the left and the data on the right.
This mode is useful if the data wouldn’t fit on the screen in the normal horizontal mode.
Expanded mode is supported by all four output modes.
null The second argument is a string that should be printed whenever a field is null. The default
is not to print anything, which can easily be mistaken for, say, an empty string. Thus, one
might choose to write \pset null ’(null)’.
fieldsep Specifies the field separator to be used in unaligned output mode. That way one can
create, for example, tab- or comma-separated output, which other programs might prefer. To
set a tab as field separator, type \pset fieldsep ’\t’. The default field separator is pipe ( | ).
recordsep Specifies the record (line) separator to use in unaligned output mode. The default is a
newline character.
tuples_only (or t) Toggles between tuples only and full display. Full display may show extra
information such as column headers, titles, and various footers. In tuples only mode, only
actual table data is shown.
title [ text ] Sets the table title for any subsequently printed tables. This can be used to give
your output descriptive tags. If no argument is given, the title is unset.
Note: This formerly only affected HTML mode. You can now set titles in any output format.

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tableattr (or T) [ text ] Allows you to specify any attributes to be placed inside the HTML table
tag. This could for example be cellpadding or bgcolor. Note that you probably donâ&#x20AC;&#x2122;t want to
specify border here, as that is already taken care of by \pset border.
pager Toggles the list of a pager to do table output. If the environment variable PAGER is set,
the output is piped to the specified program. Otherwise more is used.
In any case, psql only uses the pager if it seems appropriate. That means among other
things that the output is to a terminal and that the table would normally not fit on the screen.
Because of the modular nature of the printing routines it is not always possible to predict
the number of lines that will actually be printed. For that reason psql might not appear very
discriminating about when to use the pager and when not to.
Illustrations on how these different formats look can be seen in the Examples section. Tip: There
are various shortcut commands for \pset. See \a, \C, \H, \t, \T, and \x.
Note: It is an error to call \pset without arguments. In the future this call might show the
current status of all printing options.
\q Quit the psql program.
\qecho text [ ... ] This command is identical to \echo except that all output will be written to the
query output channel, as set by \o.
\r Resets (clears) the query buffer.
\s [ filename ] Print or save the command line history to filename. If filename is omitted, the
history is written to the standard output. This option is only available if psql is configured to
use the GNU history library.
Note: As of psql version 7.0 it is no longer necessary to save the command history, since that
will be done automatically on program termination. The history is also loaded automatically
every time psql starts up.
\set [ name [ value [ ... ]]] Sets the internal variable name to value or, if more than one value is
given, to the concatenation of all of them. If no second argument is given, the variable is just
set with no value. To unset a variable, use the \unset command.
Valid variable names can contain characters, digits, and underscores. See the section about
psql variables for details.
Although you are welcome to set any variable to anything you want, psql treats several
variables as special. They are documented in the section about variables.
Note: This command is totally separate from the SQL command SET.
\t Toggles the display of output column name headings and row count footer. This command is
equivalent to \pset tuples_only and is provided for convenience.

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\T table_options Allows you to specify options to be placed within the table tag in HTML tabular
output mode. This command is equivalent to \pset tableattr table_options.
\w {filename | |command} Outputs the current query buffer to the file filename or pipes it to
the Unix command command.
\x Toggles extended row format mode. As such it is equivalent to \pset expanded.
\z [ pattern ] Produces a list of all tables in the database with their appropriate access permissions
listed. If an argument is given it is taken as a regular expression which limits the listing to
those tables which match it.
test=> \z
Access permissions for database "test"
Relation | Access permissions
----------+------------------------------------my_table | { "=r","joe=arwR", "group staff=ar"}
(1 row )

Read this as follows:
• "=r": PUBLIC has read (SELECT ) permission on the table.
• "joe=arwR": User joe has read, write (UPDATE, DELETE ), append (INSERT ) permissions, and permission to create rules on the table.
• "group staff=ar": Group staff has SELECT and INSERT permission.
The commands GRANT and REVOKE are used to set access permissions.
\! [ command ] Escapes to a separate Unix shell or executes the Unix command command. The
arguments are not further interpreted, the shell will see them as is.
\? Get help information about the backslash ( \ ) commands.

Command-line Options
If so configured, psql understands both standard Unix short options, and GNU-style long options.
The latter are not available on all systems.
-a, –echo-all Print all the lines to the screen as they are read. This is more useful for script
processing rather than interactive mode. This is equivalent to setting the variable ECHO to
all.
-A, –no-align Switches to unaligned output mode. (The default output mode is otherwise aligned.)

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-c, –command query Specifies that psql is to execute one query string, query, and then exit. This
is useful in shell scripts.
query must be either a query string that is completely parseable by the backend (i.e., it
contains no psql specific features), or it is a single backslash command. Thus you cannot mix
SQL and psql meta-commands. To achieve that, you could pipe the string into psql, like this:
echo "\x \ \ select * from foo;" | psql.

-d, –dbname dbname Specifies the name of the database to connect to. This is equivalent to
specifying dbname as the first non-option argument on the command line.
-e, –echo-queries Show all queries that are sent to the backend. This is equivalent to setting the
variable ECHO to queries.
-E, –echo-hidden Echoes the actual queries generated by \d and other backslash commands. You
can use this if you wish to include similar functionality into your own programs. This is
equivalent to setting the variable ECHO_HIDDEN from within psql.
-f, –file filename Use the file filename as the source of queries instead of reading queries interactively. After the file is processed, psql terminates. This is in many ways equivalent to the
internal command \i.
Using this option is subtly different from writing psql < filename. In general, both will
do what you expect, but using -f enables some nice features such as error messages with
line numbers. There is also a slight chance that using this option will reduce the start-up
overhead. On the other hand, the variant using the shell’s input redirection is (in theory)
guaranteed to yield exactly the same output that you would have gotten had you entered
everything by hand.
-F, –field-separator separator Use separator as the field separator. This is equivalent to \pset
fieldsep or \f.
-h, –host hostname Specifies the host name of the machine on which the postmaster is running.
Without this option, communication is performed using local Unix domain sockets.
-H, –html Turns on HTML tabular output. This is equivalent to \pset format html or the \H
command.
-l, –list Lists all available databases, then exits. Other non-connection options are ignored. This
is similar to the internal command \list.
-o, –output filename Put all query output into file filename. This is equivalent to the command
\o.

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413

-p, –port port Specifies the TCP/IP port or, by omission, the local Unix domain socket file extension on which the postmaster is listening for connections. Defaults to the value of the
PGPORT environment variable or, if not set, to the port specified at compile time, usually
5432.
-P, –pset assignment Allows you to specify printing options in the style of \pset on the command
line. Note that here you have to separate name and value with an equal sign instead of a
space. Thus to set the output format to LATEX, you could write -P format=latex.
-q Specifies that psql should do its work quietly. By default, it prints welcome messages and
various informational output. If this option is used, none of this happens. This is useful with
the -c option. Within psql you can also set the QUIET variable to achieve the same effect.
-R, –record-separator separator Use separator as the record separator. This is equivalent to
the \pset recordsep command.
-s, –single-step Run in single-step mode. That means the user is prompted before each query is
sent to the backend, with the option to cancel execution as well. Use this to debug scripts.
-S, –single-line Runs in single-line mode where a newline terminates a query, as a semicolon
does.
Note: This mode is provided for those who insist on it, but you are not necessarily encouraged
to use it. In particular, if you mix SQL and meta-commands on a line the order of execution
might not always be clear to the inexperienced user.
-t, –tuples-only Turn off printing of column names and result row count footers, etc. It is
completely equivalent to the \t meta-command.
-T, –table-attr table_options Allows you to specify options to be placed within the HTML table
tag. See \pset for details.
-u Makes psql prompt for the user name and password before connecting to the database.
This option is deprecated, as it is conceptually flawed. (Prompting for a non-default user
name and prompting for a password because the backend requires it are really two different
things.) You are encouraged to look at the -U and -W options instead.
-U, –username username Connects to the database as the user username instead of the default.
(You must have permission to do so, of course.)
-v, –variable, –set assignment Performs a variable assignment, like the \set internal command.
Note that you must separate name and value, if any, by an equal sign on the command line.
To unset a variable, leave off the equal sign. These assignments are done during a very early
state of start-up, so variables reserved for internal purposes might get overwritten later.
-V, –version Shows the psql version.

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-W, –password Requests that psql should prompt for a password before connecting to a database.
This will remain set for the entire session, even if you change the database connection with
the meta-command \connect.
As of version 7.0, psql automatically issues a password prompt whenever the backend requests password authentication. Because this is currently based on a hack, the automatic
recognition might mysteriously fail, hence this option to force a prompt. If no password
prompt is issued and the backend requires password authentication the connection attempt
will fail.
-x, –expanded Turns on extended row format mode. This is equivalent to the command \x.
-X, –no-psqlrc Do not read the start-up file /.psqlrc.
-?, –help Shows help about psql command line arguments.

Advanced features
Variables
psql provides variable substitution features similar to common Unix command shells. This feature
is new and not very sophisticated, yet, but there are plans to expand it in the future. Variables are
simply name/value pairs, where the value can be any string of any length. To set variables, use the
psql meta-command \set:
testdb=>

\set foo bar

sets the variable foo to the value bar. To retrieve the content of the variable, precede the name
with a colon and use it as the argument of any slash command:
testdb=>
bar

\echo :foo

Note: The arguments of \set are subject to the same substitution rules as with other commands.
Thus you can construct interesting references such as \set :foo ’something’ and get soft links or
variable variables, of Perl or PHP fame, respectively. Unfortunately (or fortunately?), there is no
way to do anything useful with these constructs. On the other hand, \set bar :foo is a perfectly
valid way to copy a variable.
If you call \set without a second argument, the variable is simply set, but has no value. To
unset (or delete) a variable, use the command \unset.
psql’s internal variable names can consist of letters, numbers, and underscores in any order
and any number of them. A number of regular variables are treated specially by psql. They
indicate certain option settings that can be changed at runtime by altering the value of the variable
or represent some state of the application. Although you can use these variables for any other

D.73. PSQL

415

purpose, this is not recommended, as the program behavior might grow really strange really
quickly. By convention, all specially treated variables consist of all upper-case letters (and possibly
numbers and underscores). To ensure maximum compatibility in the future, avoid such variables.
A list of all specially treated variables follows.
DBNAME The name of the database you are currently connected to. This is set every time you
connect to a database (including program start-up), but can be unset.
ECHO If set to all, all lines entered or from a script are written to the standard output before
they are parsed or executed. To specify this on program start-up, use the switch -a. If set to
queries, psql merely prints all queries as they are sent to the backend. The option for this is
-e.
ECHO_HIDDEN When this variable is set and a backslash command queries the database, the
query is first shown. This way you can study the Postgres internals and provide similar
functionality in your own programs. If you set the variable to the value noexec, the queries
are just shown but are not actually sent to the backend and executed.
ENCODING The current client multibyte encoding. If you are not set up to use multibyte
characters, this variable will always contain SQL_ASCII.
HISTCONTROL If this variable is set to ignorespace, lines which begin with a space are not
entered into the history list. If set to a value of ignoredups, lines matching the previous
history line are not entered. A value of ignoreboth combines the two options. If unset, or if
set to any other value than those above, all lines read in interactive mode are saved on the
history list.
Note: This feature was shamelessly plagiarized from bash.
HISTSIZE The number of commands to store in the command history. The default value is 500.
Note: This feature was shamelessly plagiarized from bash.
HOST The database server host you are currently connected to. This is set every time you
connect to a database (including program start-up), but can be unset.
IGNOREEOF If unset, sending an EOF character (usually Control-D) to an interactive session
of psql will terminate the application. If set to a numeric value, that many EOF characters
are ignored before the application terminates. If the variable is set but has no numeric value,
the default is 10.
Note: This feature was shamelessly plagiarized from bash.
LASTOID The value of the last affected oid, as returned from an INSERT or lo_insert command.
This variable is only guaranteed to be valid until after the result of the next SQL command
has been displayed.

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LO_TRANSACTION If you use the Postgres large object interface to specially store data that
does not fit into one tuple, all the operations must be contained in a transaction block. (See
the documentation of the large object interface for more information.) Since psql has no way
to tell if you already have a transaction in progress when you call one of its internal commands
(\lo_export, \lo_import, \lo_unlink) it must take some arbitrary action. This action could
either be to roll back any transaction that might already be in progress, or to commit any
such transaction, or to do nothing at all. In the last case you must provide your own BEGIN
TRANSACTION / COMMIT block or the results will be unpredictable (usually resulting in
the desired actionâ&#x20AC;&#x2122;s not being performed in any case).
To choose what you want to do you set this variable to one of rollback, commit, or nothing.
The default is to roll back the transaction. If you just want to load one or a few objects this is
fine. However, if you intend to transfer many large objects, it might be advisable to provide
one explicit transaction block around all commands.
ON_ERROR_STOP By default, if non-interactive scripts encounter an error, such as a malformed
SQL query or internal meta-command, processing continues. This has been the traditional
behavior of psql but it is sometimes not desirable. If this variable is set, script processing
will immediately terminate. If the script was called from another script it will terminate in
the same fashion. If the outermost script was not called from an interactive psql session but
rather using the -f option, psql will return error code 3, to distinguish this case from fatal
error conditions (error code 1).
PORT The database server port to which you are currently connected. This is set every time
you connect to a database (including program start-up), but can be unset.
PROMPT1, PROMPT2, PROMPT3 These specify what the prompt psql issues is supposed to
look like. See Prompting below.
QUIET This variable is equivalent to the command line option -q. It is probably not too useful in
interactive mode.
SINGLELINE This variable is set by the command line option -S. You can unset or reset it at
run time.
SINGLESTEP This variable is equivalent to the command line option -s.
USER The database user you are currently connected as. This is set every time you connect to
a database (including program start-up), but can be unset.
SQL Interpolation
An additional useful feature of psql variables is that you can substitute (interpolate) them into
regular SQL statements. The syntax for this is again to prepend the variable name with a colon (:).

D.73. PSQL
testdb=>
testdb=>

417
\set foo ’my_table’
SELECT * FROM :foo;

would then query the table my_table. The value of the variable is copied literally, so it can even
contain unbalanced quotes or backslash commands. You must make sure that it makes sense where
you put it. Variable interpolation will not be performed into quoted SQL entities.
A popular application of this facility is to refer to the last inserted OID in subsequent statements
to build a foreign key scenario. Another possible use of this mechanism is to copy the contents of
a file into a field. First load the file into a variable and then proceed as above.
testdb=>
testdb=>

One possible problem with this approach is that my_file.txt might contain single quotes. These
need to be escaped so that they don’t cause a syntax error when the third line is processed. This
could be done with the program sed:
testdb=>

\set content ‘sed -e "s/’/\\\\\\’/g" < my_file.txt‘

Observe the correct number of backslashes (6)! You can resolve it this way: After psql has parsed
this line, it passes sed -e "s/’/\ \ \’/g" < my_file.txt to the shell. The shell will do it’s own
thing inside the double quotes and execute sed with the arguments -e and s/’/\ \’/g. When sed parses
this it will replace the two backslashes with a single one and then do the substitution. Perhaps at
one point you thought it was great that all Unix commands use the same escape character. And
this is ignoring the fact that you might have to escape all backslashes as well because SQL text
constants are also subject to certain interpretations. In that case you might be better off preparing
the file externally.
Since colons may legally appear in queries, the following rule applies: If the variable is not
set, the character sequence colon+name is not changed. In any case you can escape a colon with
a backslash to protect it from interpretation. (The colon syntax for variables is standard SQL for
embedded query languages, such as ecpg. The colon syntax for array slices and type casts are
Postgres extensions, hence the conflict.)
Prompting
The prompts psql issues can be customized to your preference. The three variables PROMPT1,
PROMPT2, and PROMPT3 contain strings and special escape sequences that describe the appearance of the prompt. Prompt 1 is the normal prompt that is issued when psql requests a new
query. Prompt 2 is issued when more input is expected during query input because the query was
not terminated with a semicolon or a quote was not closed. Prompt 3 is issued when you run an
SQL COPY command and you are expected to type in the tuples on the terminal.

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The value of the respective prompt variable is printed literally, except where a percent sign
(%) is encountered. Depending on the next character, certain other text is substituted instead.
Defined substitutions are:
%M The full hostname (with domain name) of the database server (or localhost if hostname
information is not available).
%m The hostname of the database server, truncated after the first dot.
%> The port number at which the database server is listening.
%n The username you are connected as (not your local system user name).
%/ The name of the current database.
% Like %/, but the output is tilde (˜) if the database is your default database.
%# If the current user is a database superuser, then a #, otherwise a >.
%R In prompt 1 normally =, but ˆ if in single-line mode, and ! if the session is disconnected from
the database (which can happen if \connect fails). In prompt 2 the sequence is replaced by
-*, a single quote, or a double quote, depending on whether psql expects more input because
the query wasn’t terminated yet, because you are inside a /* ... */ comment, or because you
are inside a quote. In prompt 3 the sequence doesn’t resolve to anything.
%digits If digits starts with 0x the rest of the characters are interpreted as a hexadecimal digit and
the character with the corresponding code is substituted. If the first digit is 0 the characters
are interpreted as on octal number and the corresponding character is substituted. Otherwise
a decimal number is assumed.
%:name: The value of the psql, variable name. See the section Variables for details.
%‘command‘ The output of command, similar to ordinary back-tick substitution.
To insert a percent sign into your prompt, write %%. The default prompts are equivalent to
’%/%R%# ’ for prompts 1 and 2, and ’> > ’ for prompt 3. Note: This feature was shamelessly
plagiarized from tcsh.
Miscellaneous
psql returns 0 to the shell if it finished normally, 1 if a fatal error of its own (out of memory, file not
found) occurs, 2 if the connection to the backend went bad and the session is not interactive, and
3 if an error occurred in a script and the variable ON_ERROR_STOP was set.
Before starting up, psql attempts to read and execute commands from the file $HOME/.psqlrc.
It could be used to set up the client or the server to taste (using the \set and SET commands).

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419

GNU readline
psql supports the readline and history libraries for convenient line editing and retrieval. The
command history is stored in a file named .psql_history in your home directory and is reloaded
when psql starts up. Tab-completion is also supported, although the completion logic makes no
claim to be an SQL parser. When available, psql is automatically built to use these features. If for
some reason you do not like the tab completion, you can turn if off by putting this in a file named
.inputrc in your home directory:
$if psql
set disable-completion on
$endif

(This is not a psql but a readline feature. Read its documentation for further details.)
If you have the readline library installed but psql does not seem to use it, you must make
sure that Postgresâ&#x20AC;&#x2122;s top-level configure script finds it. configure needs to find both the library
libreadline.a (or a shared library equivalent) and the header files readline.h and history.h (or
readline/readline.h and readline/history.h) in appropriate directories. If you have the library and
header files installed in an obscure place you must tell configure about them, for example:
$ ./configure --with-includes=/opt/gnu/include --with-libs=/opt/gnu/lib ...

Then you have to recompile psql (not necessarily the entire code tree).
The GNU readline library can be obtained from the GNU projectâ&#x20AC;&#x2122;s FTP server at ftp://ftp.gnu.org.

Examples
Note: This section only shows a few examples specific to psql. If you want to learn SQL or get
familiar with Postgres, you might wish to read the Tutorial that is included in the distribution.
The first example shows how to spread a query over several lines of input. Notice the changing
prompt:
testdb=>
testdb(>
testdb(>
testdb->
CREATE

Alternatively, use the short commands:
peter@localhost testdb=> \a \t \x
Output format is aligned. Tuples only is off. Expanded display is on.
peter@localhost testdb=> SELECT * FROM my_table;
-[ RECORD 1 ]- first | 1 second | one
-[ RECORD 2 ]- first | 2 second | two
-[ RECORD 3 ]- first | 3 second | three
-[ RECORD 4 ]- first | 4 second | four

Appendix
Bugs and Issues
â&#x20AC;˘ In some earlier life psql allowed the first argument to start directly after the (single-letter)
command. For compatibility this is still supported to some extent but I am not going to
explain the details here as this use is discouraged. But if you get strange messages, keep
this in mind. For example:
testdb=>

\foo

Field separator is "oo", which is perhaps not what one would expect.
â&#x20AC;˘ psql only works smoothly with servers of the same version. That does not mean other
combinations will fail outright, but subtle and not-so-subtle problems might come up.

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422

• Pressing Control-C during a copy in (data sent to the server) doesn’t show the most ideal of
behaviors. If you get a message such as PQexec: you gotta get out of a COPY state yourself
—simply reset the connection by entering \c - -.

D.74

REINDEX

Name
REINDEX — Recover corrupted system indexes under stand-alone Postgres
REINDEX { TABLE | DATABASE | INDEX } name [ FORCE ]
Inputs
TABLE Recreate all indexes of a specified table.
DATABASE Recreate all system indexes of a specified database.
INDEX Recreate a specified index.
name The name of the specific table/database/index to be be reindexed.
FORCE Recreate indexes forcedly. Without this keyword REINDEX does nothing unless target
indexes are invalidated.
Outputs
REINDEX Message returned if the table is successfully reindexed.

Description
REINDEX is used to recover corrupted system indexes. In order to run REINDEX command,
postmaster must be shut down and stand-alone Postgres should be started instead with options -O
and -P (an option to ignore system indexes). Note that we couldn’t rely on system indexes for the
recovery of system indexes.

Name
RESET â&#x20AC;&#x201D; Restores run-time parameters for session to default values
RESET variable
Inputs
variable Refer to SET for more information on available variables.
Outputs
RESET VARIABLE Message returned if variable is successfully reset to its default value.

Description
RESET restores variables to their default values. Refer to SET for details on allowed values and
defaults. RESET is an alternate form for SET variable = DEFAULT.
Notes
See also SET and SHOW to manipulate variable values.

Usage
Set DateStyle to its default value:
RESET DateStyle;

Set Geqo to its default value:
RESET GEQO;

Compatibility
SQL92
There is no RESET in SQL92.

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424

D.76

REVOKE

Name
REVOKE — Revokes access privilege from a user, a group or all users.
REVOKE privilege [, ...]
ON object [, ...]
FROM { PUBLIC | GROUP groupname | username }
Inputs
privilege The possible privileges are:
SELECT Privilege to access all of the columns of a specific table/view.
INSERT Privilege to insert data into all columns of a specific table.
UPDATE Privilege to update all columns of a specific table.
DELETE Privilege to delete rows from a specific table.
RULE Privilege to define rules on table/view. (See CREATE RULE ).
ALL Rescind all privileges.
object The name of an object from which to revoke access. The possible objects are:
• table
• view
• sequence
group The name of a group from whom to revoke privileges.
username The name of a user from whom revoke privileges. Use the PUBLIC keyword to specify
all users.
PUBLIC Rescind the specified privilege(s) for all users.
Outputs
CHANGE Message returned if successfully.
ERROR Message returned if object is not available or impossible to revoke privileges from a
group or users.

Tip: Currently, to create a GROUP you have to insert data manually into table pg_group as:
INSERT INTO pg_group VALUES (’todos’);
CREATE USER miriam IN GROUP todos;

Usage
Revoke insert privilege from all users on table films:
REVOKE INSERT ON films FROM PUBLIC;

Revoke all privileges from user manuel on view kinds:
REVOKE ALL ON kinds FROM manuel;

APPENDIX D. REFERENCE MANUAL

426

Compatibility
SQL92
The SQL92 syntax for REVOKE has additional capabilities for rescinding privileges, including
those on individual columns in tables:
REVOKE { SELECT | DELETE | USAGE | ALL PRIVILEGES } […] ON object
FROM { PUBLIC | username [, ... } { RESTRICT | CASCADE } REVOKE { INSERT |
UPDATE | REFERENCES } [, ...] [ ( column [, ...] ) ] ON object
FROM { PUBLIC | username [, ...] } { RESTRICT | CASCADE } ] Refer to GRANT for
details on individual fields.
REVOKE GRANT OPTION FOR privilege [, ... ON object
FROM { PUBLIC | username [, ...] } { RESTRICT | CASCADE } ] Rescinds authority for
a user to grant the specified privilege to others. Refer to GRANT for details on individual
fields.
The possible objects are:
If user1 gives a privilege WITH GRANT OPTION to user2, and user2 gives it to user3 then
user1 can revoke this privilege in cascade using the CASCADE keyword.
If user1 gives a privilege WITH GRANT OPTION to user2, and user2 gives it to user3, then if
user1 tries to revoke this privilege it fails if he specifies the RESTRICT keyword.

D.77

ROLLBACK

Name
ROLLBACK — Aborts the current transaction
ROLLBACK [ WORK | TRANSACTION ]
Inputs
None.
Outputs
ABORT Message returned if successful.
NOTICE: ROLLBACK: no transaction in progress If there is not any transaction currently
in progress.

D.78. SELECT

427

Description
ROLLBACK rolls back the current transaction and causes all the updates made by the transaction
to be discarded.
Notes
Use COMMIT to successfully terminate a transaction. ABORT is a synonym for ROLLBACK.

Usage
To abort all changes:
ROLLBACK WORK;

Compatibility
SQL92
SQL92 only specifies the two forms ROLLBACK and ROLLBACK WORK. Otherwise full compatibility.

Inputs
expression The name of a table’s column or an expression.
name Specifies another name for a column or an expression using the AS clause. This name is
primarily used to label the column for display. It can also be used to refer to the column’s
value in ORDER BY and GROUP BY clauses. But the name cannot be used in the WHERE
or HAVING clauses; write out the expression instead.
TEMPORARY, TEMP If TEMPORARY or TEMP is specified, the table is created unique to this
session, and is automatically dropped on session exit.
new_table If the INTO TABLE clause is specified, the result of the query will be stored in a new
table with the indicated name. The target table (new_table ) will be created automatically and
must not exist before this command. Refer to SELECT INTO for more information. Note:
The CREATE TABLE AS statement will also create a new table from a select query.
table The name of an existing table referenced by the FROM clause.
alias An alternate name for the preceding table. It is used for brevity or to eliminate ambiguity
for joins within a single table.
condition A boolean expression giving a result of true or false. See the WHERE clause.
column The name of a table’s column.
select A select statement with all features except the ORDER BY and LIMIT clauses.
Outputs
Rows The complete set of rows resulting from the query specification.
count The count of rows returned by the query.

Description
SELECT will return rows from one or more tables. Candidates for selection are rows which satisfy
the WHERE condition; if WHERE is omitted, all rows are candidates. (See WHERE Clause.)
DISTINCT will eliminate duplicate rows from the result. ALL (the default) will return all
candidate rows, including duplicates.
DISTINCT ON eliminates rows that match on all the specified expressions, keeping only the
first row of each set of duplicates. The DISTINCT ON expressions are interpreted using the same
rules as for ORDER BY items; see below. Note that "the first row" of each set is unpredictable
unless ORDER BY is used to ensure that the desired row appears first. For example,

D.78. SELECT

429

SELECT DISTINCT ON (location) location, time, report
FROM weatherReports
ORDER BY location, time DESC;

retrieves the most recent weather report for each location. But if we had not used ORDER BY to
force descending order of time values for each location, weâ&#x20AC;&#x2122;d have gotten a report of unpredictable
age for each location.
The GROUP BY clause allows a user to divide a table into groups of rows that match on one or
more values. (See GROUP BY Clause.)
The HAVING clause allows selection of only those groups of rows meeting the specified
condition. (See HAVING Clause.)
The ORDER BY clause causes the returned rows to be sorted in a specified order. If ORDER
BY is not given, the rows are returned in whatever order the system finds cheapest to produce.
(See ORDER BY Clause.)
The UNION operator allows the result to be the collection of rows returned by the queries
involved. (See UNION Clause.)
The INTERSECT operator gives you the rows that are common to both queries. (See INTERSECT Clause.)
The EXCEPT operator gives you the rows returned by the first query but not the second query.
(See EXCEPT Clause.)
The FOR UPDATE clause allows the SELECT statement to perform exclusive locking of
selected rows.
The LIMIT clause allows a subset of the rows produced by the query to be returned to the
user. (See LIMIT Clause.)
You must have SELECT privilege to a table to read its values (See the GRANT /REVOKE
statements).
WHERE Clause
The optional WHERE condition has the general form:
WHERE boolean_expr

boolean_expr can consist of any expression which evaluates to a boolean value. In many cases, this
expression will be:
expr cond_op expr

or
log_op expr

where cond_op can be one of: =, <, <=, >, >= or <>, a conditional operator like ALL, ANY,
IN, LIKE, or a locally defined operator, and log_op can be one of: AND, OR, NOT. SELECT will
ignore all rows for which the WHERE condition does not return TRUE.

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430
GROUP BY Clause

GROUP BY specifies a grouped table derived by the application of this clause:
GROUP BY column [, ...]

GROUP BY will condense into a single row all selected rows that share the same values for the
grouped columns. Aggregate functions, if any, are computed across all rows making up each group,
producing a separate value for each group (whereas without GROUP BY, an aggregate produces
a single value computed across all the selected rows). When GROUP BY is present, it is not
valid for the SELECT output expression(s) to refer to ungrouped columns except within aggregate
functions, since there would be more than one possible value to return for an ungrouped column.
An item in GROUP BY can also be the name or ordinal number of an output column (SELECT
expression), or it can be an arbitrary expression formed from input-column values. In case of
ambiguity, a GROUP BY name will be interpreted as an input-column name rather than an output
column name.
HAVING Clause
The optional HAVING condition has the general form:
HAVING cond_expr

where cond_expr is the same as specified for the WHERE clause.
HAVING specifies a grouped table derived by the elimination of group rows that do not satisfy the cond_expr. HAVING is different from WHERE: WHERE filters individual rows before
application of GROUP BY, while HAVING filters group rows created by GROUP BY.
Each column referenced in cond_expr shall unambiguously reference a grouping column, unless
the reference appears within an aggregate function.
ORDER BY Clause
ORDER BY column [ ASC | DESC ] [, ...]
column can be either a result column name or an ordinal number.
The ordinal numbers refers to the ordinal (left-to-right) position of the result column. This
feature makes it possible to define an ordering on the basis of a column that does not have a proper
name. This is never absolutely necessary because it is always possible to assign a name to a result
column using the AS clause, e.g.:
SELECT title, date_prod + 1 AS newlen
FROM films
ORDER BY newlen;

D.78. SELECT

431

It is also possible to ORDER BY arbitrary expressions (an extension to SQL92), including fields
that do not appear in the SELECT result list. Thus the following statement is legal:
SELECT name
FROM distributors
ORDER BY code;

Note that if an ORDER BY item is a simple name that matches both a result column name and an
input column name, ORDER BY will interpret it as the result column name. This is the opposite
of the choice that GROUP BY will make in the same situation. This inconsistency is mandated by
the SQL92 standard.
Optionally one may add the keyword DESC (descending) or ASC (ascending) after each column
name in the ORDER BY clause. If not specified, ASC is assumed by default. Alternatively, a
specific ordering operator name may be specified. ASC is equivalent to USING ’< ’ and DESC is
equivalent to USING ’> ’.
UNION Clause
table_query UNION [ ALL ] table_query
[ ORDER BY column [ ASC | DESC ] [, ...] ] where table_query specifies any select expression
without an ORDER BY or LIMIT clause.
The UNION operator allows the result to be the collection of rows returned by the queries
involved. The two SELECTs that represent the direct operands of the UNION must produce the
same number of columns, and corresponding columns must be of compatible data types.
By default, the result of UNION does not contain any duplicate rows unless the ALL clause is
specified.
Multiple UNION operators in the same SELECT statement are evaluated left to right. Note
that the ALL keyword is not global in nature, being applied only for the current pair of table results.
INTERSECT Clause
table_query INTERSECT table_query
[ ORDER BY column [ ASC | DESC ] [, ...] ] where table_query specifies any select expression
without an ORDER BY or LIMIT clause.
The INTERSECT operator gives you the rows that are common to both queries. The two
SELECTs that represent the direct operands of the INTERSECT must produce the same number
of columns, and corresponding columns must be of compatible data types.
Multiple INTERSECT operators in the same SELECT statement are evaluated left to right,
unless parentheses dictate otherwise.

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432
EXCEPT Clause

table_query EXCEPT table_query
[ ORDER BY column [ ASC | DESC ] [, ...] ] where table_query specifies any select expression
without an ORDER BY or LIMIT clause.
The EXCEPT operator gives you the rows returned by the first query but not the second query.
The two SELECTs that represent the direct operands of the EXCEPT must produce the same
number of columns, and corresponding columns must be of compatible data types.
Multiple EXCEPT operators in the same SELECT statement are evaluated left to right, unless
parentheses dictate otherwise.
LIMIT Clause
LIMIT { count | ALL } [ { OFFSET | , } start ] OFFSET start
where count specifies the maximum number of rows to return, and start specifies the number
of rows to skip before starting to return rows.
LIMIT allows you to retrieve just a portion of the rows that are generated by the rest of the
query. If a limit count is given, no more than that many rows will be returned. If an offset is given,
that many rows will be skipped before starting to return rows.
When using LIMIT, it is a good idea to use an ORDER BY clause that constrains the result rows
into a unique order. Otherwise you will get an unpredictable subset of the query’s rows —you may
be asking for the tenth through twentieth rows, but tenth through twentieth in what ordering? You
don’t know what ordering, unless you specified ORDER BY.
As of Postgres 7.0, the query optimizer takes LIMIT into account when generating a query
plan, so you are very likely to get different plans (yielding different row orders) depending on
what you give for LIMIT and OFFSET. Thus, using different LIMIT/OFFSET values to select
different subsets of a query result will give inconsistent results unless you enforce a predictable
result ordering with ORDER BY. This is not a bug; it is an inherent consequence of the fact that
SQL does not promise to deliver the results of a query in any particular order unless ORDER BY
is used to constrain the order.

To sum the column len of all films and group the results by kind:
SELECT kind, SUM(len) AS total
FROM films
GROUP BY kind;
kind | total
----------+------Action
| 07:34
Comedy
| 02:58
Drama
| 14:28
Musical | 06:42
Romantic | 04:38
(5 rows)

To sum the column len of all films, group the results by kind and show those group totals that are
less than 5 hours:
SELECT kind, SUM(len) AS total
FROM films
GROUP BY kind
HAVING SUM(len) < INTERVAL ’5 hour’;
kind
| total
----------+------Comedy
| 02:58
Romantic | 04:38

This example shows how to obtain the union of the tables distributors and actors, restricting the
results to those that begin with letter W in each table. Only distinct rows are wanted, so the ALL
keyword is omitted:
distributors:
did |
name
-----+-------------108 | Westward
111 | Walt Disney
112 | Warner Bros.
...

Compatibility
Extensions
Postgres allows one to omit the FROM clause from a query. This feature was retained from the
original PostQuel query language:
SELECT distributors.*
WHERE name = ’Westwood’;
did | name
-----+---------108 | Westward

SQL92
SELECT Clause
In the SQL92 standard, the optional keyword "AS" is just noise and can be omitted without
affecting the meaning. The Postgres parser requires this keyword when renaming columns because
the type extensibility features lead to parsing ambiguities in this context.
The DISTINCT ON phrase is not part of SQL92. Nor are LIMIT and OFFSET.
In SQL92, an ORDER BY clause may only use result column names or numbers, while a
GROUP BY clause may only use input column names. Postgres extends each of these clauses
to allow the other choice as well (but it uses the standard’s interpretation if there is ambiguity).
Postgres also allows both clauses to specify arbitrary expressions. Note that names appearing in
an expression will always be taken as input-column names, not as result-column names.

Description
SELECT INTO creates a new table from the results of a query. Typically, this query draws data
from an existing table, but any SQL query is allowed. Note: CREATE TABLE AS is functionally
equivalent to the SELECT INTO command.

D.80. SET

D.80

437

SET

Name
SET — Set run-time parameters for session
SET variable { TO | = } { value | ’value ’ | DEFAULT }
SET CONSTRAINTS { ALL | constraintlist } mode
SET TIME ZONE { ’timezone ’ | LOCAL | DEFAULT }
SET TRANSACTION ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE }
Inputs
variable Settable global parameter.
value New value of parameter. DEFAULT can be used to specify resetting the parameter to its
default value. Lists of strings are allowed, but more complex constructs may need to be
single or double quoted.
The possible variables and allowed values are:
CLIENT_ENCODING | NAMES Sets the multibyte client encoding. Parameters are:
value Sets the multibyte client encoding to value. The specified encoding must be supported
by the backend.
This option is only available if MULTIBYTE support was enabled during the configure step
of building Postgres.
DATESTYLE Set the date/time representation style. Affects the output format, and in some
cases it can affect the interpretation of input.
ISO use ISO 8601-style dates and times
SQL use Oracle/Ingres-style dates and times
Postgres use traditional Postgres format
European use dd/mm/yyyy for numeric date representations.
NonEuropean use mm/dd/yyyy for numeric date representations.
German use dd.mm.yyyy for numeric date representations.
US same as NonEuropean
DEFAULT restores the default values (ISO)
Date format initialization may be done by:

APPENDIX D. REFERENCE MANUAL

438

• Setting the PGDATESTYLE environment variable. If PGDATESTYLE is set in the
frontend environment of a client based on libpq, libpq will automatically set DATESTYLE
to the value of PGDATESTYLE during connection start-up.
• Running postmaster using the option -o -e to set dates to the European convention.
SEED Sets the internal seed for the random number generator.
value The value for the seed to be used by the random catalog function. Significant values
are floating point numbers between 0 and 1, which are then multiplied by RAND_MAX.
This product will silently overflow if a number outside the range is used.
The seed can also be set by invoking the setseed SQL function:
SELECT setseed(value);

This option is only available if MULTIBYTE support was enabled during the configure step
of building Postgres.
SERVER_ENCODING Sets the multibyte server encoding to:
value The identifying value for the server encoding.
This option is only available if MULTIBYTE support was enabled during the configure step
of building Postgres.
CONSTRAINTS SET CONSTRAINTS affects the behavior of constraint evaluation in the current
transaction. SET CONSTRAINTS, specified in SQL3, has these allowed parameters:
constraintlist Comma separated list of deferrable constraint names.
mode The constraint mode. Allowed values are DEFERRED and IMMEDIATE.
In IMMEDIATE mode, foreign key constraints are checked at the end of each query.
In DEFERRED mode, foreign key constraints marked as DEFERRABLE are checked only
at transaction commit or until its mode is explicitly set to IMMEDIATE. This is actually only
done for foreign key constraints, so it does not apply to UNIQUE or other constraints.
TIME ZONE, TIMEZONE The possible values for time zone depends on your operating system.
For example, on Linux /usr/lib/zoneinfo contains the database of time zones.
Here are some valid values for time zone:
PST8PDT set the time zone for California
Portugal set time zone for Portugal.
’Europe/Rome’ set time zone for Italy.

D.80. SET

439

DEFAULT set time zone to your local time zone (value of the TZ environment variable).
If an invalid time zone is specified, the time zone becomes GMT (on most systems anyway).
The second syntax shown above, allows one to set the time zone with a syntax similar to
SQL92 SET TIME ZONE. The LOCAL keyword is just an alternate form of DEFAULT for
SQL92 compatibility.
If the PGTZ environment variable is set in the frontend environment of a client based on libpq,
libpq will automatically set TIMEZONE to the value of PGTZ during connection start-up.
TRANSACTION ISOLATION LEVEL Sets the isolation level for the current transaction.
READ COMMITTED The current transaction queries read only rows committed before a
query began. READ COMMITTED is the default.
Note: SQL92 standard requires SERIALIZABLE to be the default isolation level.
SERIALIZABLE The current transaction queries read only rows committed before first
DML statement (SELECT/INSERT/DELETE/UPDATE/FETCH/COPY_TO ) was
executed in this transaction.
There are also several internal or optimization parameters which can be specified by the SET
command:
PG_OPTIONS Sets various backend parameters.
RANDOM_PAGE_COST Sets the optimizer’s estimate of the cost of a nonsequentially fetched
disk page. This is measured as a multiple of the cost of a sequential page fetch.
float8 Set the cost of a random page access to the specified floating-point value.
CPU_TUPLE_COST Sets the optimizer’s estimate of the cost of processing each tuple during a
query. This is measured as a fraction of the cost of a sequential page fetch.
float8 Set the cost of per-tuple CPU processing to the specified floating-point value.
CPU_INDEX_TUPLE_COST Sets the optimizer’s estimate of the cost of processing each index
tuple during an index scan. This is measured as a fraction of the cost of a sequential page
fetch.
float8 Set the cost of per-index-tuple CPU processing to the specified floating-point value.
CPU_OPERATOR_COST Sets the optimizer’s estimate of the cost of processing each operator
in a WHERE clause. This is measured as a fraction of the cost of a sequential page fetch.
float8 Set the cost of per-operator CPU processing to the specified floating-point value.

APPENDIX D. REFERENCE MANUAL

440

EFFECTIVE_CACHE_SIZE Sets the optimizer’s assumption about the effective size of the
disk cache (that is, the portion of the kernel’s disk cache that will be used for Postgres data
files). This is measured in disk pages, which are normally 8Kb apiece.
float8 Set the assumed cache size to the specified floating-point value.
ENABLE_SEQSCAN Enables or disables the planner’s use of sequential scan plan types. (It’s
not possible to suppress sequential scans entirely, but turning this variable OFF discourages
the planner from using one if there is any other method available.)
ON enables use of sequential scans (default setting).
OFF disables use of sequential scans.
ENABLE_INDEXSCAN Enables or disables the planner’s use of index scan plan types.
ON enables use of index scans (default setting).
OFF disables use of index scans.
ENABLE_TIDSCAN Enables or disables the planner’s use of TID scan plan types.
ON enables use of TID scans (default setting).
OFF disables use of TID scans.
ENABLE_SORT Enables or disables the planner’s use of explicit sort steps. (It’s not possible to
suppress explicit sorts entirely, but turning this variable OFF discourages the planner from
using one if there is any other method available.)
ON enables use of sorts (default setting).
OFF disables use of sorts.
ENABLE_NESTLOOP Enables or disables the planner’s use of nested-loop join plans. (It’s not
possible to suppress nested-loop joins entirely, but turning this variable OFF discourages
the planner from using one if there is any other method available.)
ON enables use of nested-loop joins (default setting).
OFF disables use of nested-loop joins.
ENABLE_MERGEJOIN Enables or disables the planner’s use of mergejoin plans.
ON enables use of merge joins (default setting).
OFF disables use of merge joins.
ENABLE_HASHJOIN Enables or disables the planner’s use of hashjoin plans.

D.80. SET

441

ON enables use of hash joins (default setting).
OFF disables use of hash joins.
GEQO Sets the threshold for using the genetic optimizer algorithm.
ON enables the genetic optimizer algorithm for statements with 11 or more tables. (This is
also the DEFAULT setting.)
ON=# Takes an integer argument to enable the genetic optimizer algorithm for statements
with # or more tables in the query.
OFF disables the genetic optimizer algorithm.
See the chapter on GEQO in the Programmer’s Guide for more information about query
optimization.
If the PGGEQO environment variable is set in the frontend environment of a client based on
libpq, libpq will automatically set GEQO to the value of PGGEQO during connection start-up.
KSQO Key Set Query Optimizer causes the query planner to convert queries whose WHERE
clause contains many OR’ed AND clauses (such as "WHERE (a=1 AND b=2) OR (a=2 AND
b=3) ...") into a UNION query. This method can be faster than the default implementation,
but it doesn’t necessarily give exactly the same results, since UNION implicitly adds a
SELECT DISTINCT clause to eliminate identical output rows. KSQO is commonly used
when working with products like Microsoft Access, which tend to generate queries of this
form.
ON enables this optimization.
OFF disables this optimization (default setting).
DEFAULT Equivalent to specifying SET KSQO=OFF.
The KSQO algorithm used to be absolutely essential for queries with many OR’ed AND
clauses, but in Postgres 7.0 and later the standard planner handles these queries fairly
successfully.
MAX_EXPR_DEPTH Sets the maximum expression nesting depth that the parser will accept.
The default value is high enough for any normal query, but you can raise it if you need to.
(But if you raise it too high, you run the risk of backend crashes due to stack overflow.)
integer Maximum depth.
Outputs
SET VARIABLE Message returned if successful.
NOTICE: Bad value for variable (value ) If the command fails to set the specified variable.

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442

Description
SET will modify configuration parameters for variable during a session.
Current values can be obtained using SHOW, and values can be restored to the defaults using
RESET. Parameters and values are case-insensitive. Note that the value field is always specified
as a string, so is enclosed in single quotes.
SET TIME ZONE changes the session’s default time zone offset. An SQL session always
begins with an initial default time zone offset. The SET TIME ZONE statement is used to change
the default time zone offset for the current SQL session.
Notes
The SET variable statement is a Postgres language extension.
Refer to SHOW and RESET to display or reset the current values.

Usage
Set the style of date to ISO (no quotes on the argument is required):
SET DATESTYLE TO ISO;

Enable GEQO for queries with 4 or more tables (note the use of single quotes to handle the equal
sign inside the value argument):
SET GEQO = ’ON=4’;

Set GEQO to default:
SET GEQO = DEFAULT;

Set the time zone for Berkeley, California, using double quotes to preserve the uppercase attributes
of the time zone specifier:
SET TIME ZONE "PST8PDT"; SELECT CURRENT_TIMESTAMP AS today;
today
-----------------------1998-03-31 07:41:21-08

Set the time zone for Italy (note the required single or double quotes to handle the special
characters):
SET TIME ZONE ’Europe/Rome’;
SELECT CURRENT_TIMESTAMP AS today;
today
-----------------------1998-03-31 17:41:31+02

D.81. SHOW

443

Compatibility
SQL92
There is no general SET variable in SQL92 (with the exception of SET TRANSACTION
ISOLATION LEVEL ). The SQL92 syntax for SET TIME ZONE is slightly different, allowing
only a single integer value for time zone specification:
SET TIME ZONE {

D.81

interval_value_expression | LOCAL }

SHOW

Name
SHOW â&#x20AC;&#x201D; Shows run-time parameters for session
SHOW keyword
Inputs
keyword Refer to SET for more information on available variables.
Outputs
NOTICE: variable is value Message returned if successful.
NOTICE: Unrecognized variable value Message returned if variable does not exist.
NOTICE: Time zone is unknown If the TZ or PGTZ environment variable is not set.

Description
SHOW will display the current setting of a run-time parameter during a session.
These variables can be set using the SET statement, and can be restored to the default values
using the RESET statement. Parameters and values are case-insensitive.
Notes
See also SET and RESET to manipulate variable values.

APPENDIX D. REFERENCE MANUAL

444

Usage
Show the current DateStyle setting:
SHOW DateStyle;
NOTICE: DateStyle is ISO with US (NonEuropean) conventions

Show the current genetic optimizer (geqo) setting:
SHOW GEQO;
NOTICE: GEQO is ON beginning with 11 relations

Compatibility
SQL92
There is no SHOW defined in SQL92.

D.82

TRUNCATE

Name
TRUNCATE â&#x20AC;&#x201D; Empty a table
TRUNCATE [ TABLE ] name
Inputs
name The name of the table to be truncated.
Outputs
TRUNCATE Message returned if the table is successfully truncated.

Description
TRUNCATE quickly removes all rows from a table. It has the same effect as an unqualified
DELETE but since it does not actually scan the table it is faster. This is most effective on large
tables.

Description
UNLISTEN is used to remove an existing NOTIFY registration. UNLISTEN cancels any existing
registration of the current Postgres session as a listener on the notify condition notifyname. The
special condition wildcard "*" cancels all listener registrations for the current session.
NOTIFY contains a more extensive discussion of the use of LISTEN and NOTIFY.
Notes
classname need not be a valid class name but can be any string valid as a name up to 32 characters
long.
The backend does not complain if you UNLISTEN something you were not listening for. Each
backend will automatically execute UNLISTEN * when exiting.
A restriction in some previous releases of Postgres that a classname which does not correspond
to an actual table must be enclosed in double-quotes is no longer present.

Once UNLISTEN has been executed, further NOTIFY commands will be ignored:
UNLISTEN virtual;
UNLISTEN
NOTIFY virtual;
NOTIFY
-- notice no NOTIFY event is received

Compatibility
SQL92
There is no UNLISTEN in SQL92.

D.84

UPDATE

Name
UPDATE — Replaces values of columns in a table
UPDATE table
SET col = expression [, ...]
[ FROM fromlist ]
[ WHERE condition ]
Inputs
table The name of an existing table.
column The name of a column in table.
expression A valid expression or value to assign to column.
fromlist A Postgres non-standard extension to allow columns from other tables to appear in the
WHERE condition.
condition Refer to the SELECT statement for a further description of the WHERE clause.

D.84. UPDATE

447

Outputs
UPDATE # Message returned if successful. The # means the number of rows updated. If # is
0 no rows are updated.

Description
UPDATE changes the values of the columns specified for all rows which satisfy condition. Only
the columns to be modified need appear as columns in the statement.
Array references use the same syntax found in SELECT. That is, either single array elements,
a range of array elements or the entire array may be replaced with a single query.
You must have write access to the table in order to modify it, as well as read access to any table
whose values are mentioned in the WHERE condition.

Description
VACUUM serves two purposes in Postgres as both a means to reclaim storage and also a means
to collect information for the optimizer.
VACUUM opens every class in the database, cleans out records from rolled back transactions,
and updates statistics in the system catalogs. The statistics maintained include the number of
tuples and number of pages stored in all classes.
VACUUM ANALYZE collects statistics representing the dispersion of the data in each column.
This information is valuable when several query execution paths are possible.
Running VACUUM periodically will increase the speed of the database in processing user
queries.

D.85. VACUUM

449

Notes
The open database is the target for VACUUM.
We recommend that active production databases be VACUUM-ed nightly, in order to remove
expired rows. After copying a large class into Postgres or after deleting a large number of records,
it may be a good idea to issue a VACUUM ANALYZE query. This will update the system catalogs
with the results of all recent changes, and allow the Postgres query optimizer to make better
choices in planning user queries.

Outputs
VACUUM Everything went well.
vacuumdb: Vacuum failed. Something went wrong. vacuumdb is only a wrapper script. See
VACUUM and psql for a detailed discussion of error messages and potential problems.

Description
vacuumdb is a utility for cleaning a Postgres database. vacuumdb will also generate internal
statistics used by the Postgres query optimizer.
vacuumdb is a shell script wrapper around the backend command VACUUM via the Postgres
interactive terminal psql. There is no effective difference between vacuuming databases via this
or other methods. psql must be found by the script and a database server must be running at the
targeted host. Also, any default settings and environment variables available to psql and the libpq
front-end library do apply.

Usage
To clean the database test:
$ vacuumdb test

To analyze a database named bigdb for the optimizer:
$ vacuumdb --analyze bigdb

To analyze a single column bar in table foo in a database named xyzzy for the optimizer:
$ vacuumdb --analyze --verbose --table â&#x20AC;&#x2122;foo(bar)â&#x20AC;&#x2122; xyzzy